That is a little disturbing, and i would hope that she would have talked to him if this was something she was not okay with, however depend on their sub/dom relationship and this may have added to their sex life.
For me crossing the line is when consent is boken, and then becomes rape/abuse regardless if it is violent or not. My partner is amazing at checking in with me, even if we're getting a little heated and moving into some BDSM practices, they will make sure to ask if i am okay when he notices a change in my body language. I've had other partners that did not do such, and when i no longer wanted to have sex with them because i felt uncomfortable (voicing this as well as showing it with my body), they physically restrained me from doing so - and this was before i started to practice any sort of lite BDSM play in the bedroom. That to be was abusive behavior as i was in the middle of a manic episode when it happened and it then became a full blown panic attack.
So for me abuse in the bedroom tends to occur when things start to borderline rape and feelings are not taken into account.

I agree with Pixel. The line in every situation is consent.

Oh now that is quiet scary. I have issue with beatings and such and don't think that I could deal with it.

Anything non-consensual is abusive. Anything that does not involve the /s partner being fully risk aware and absolutely consenting is wrong. Also, anything involving the Top not being fully educated, knowledgable, risk aware and safe about what they are doing is at least ignorantly neglectful and possibly abusive. After a relationship that has been broken has been repaired a punishment session or a re-establishing dominance type session wouldn't be out of the question and might help to repair rifts and rebalance the relationship but if it's done against the /s's consent then it's not ok. Period. The problem is that, as outsiders, we can't tell where that line is for anyone else. The only person who can say whether what this D/ was abusive is the pet in question. If he held her against her will, beat her and ignored her safeword or signal, then, personally, i'd consider that abusive, but it's really up to her and there's a certain level of personal responsibility on her part. Of course, i understand the mentality of the abuse victim and standing up for yourself isn't always possible. It's how abuse works and why the victims stay, but unfortunately, only the victim can break the cycle. If, for instance, she didn't use her safeword, didn't object, and went along with what he did, and he didn't break any of her negotiated limits, we might look at what he did and judge that it was wrong in our opinions, or that it would be wrong if we were the pet, but without her objection it's not for us to say that this was wrong for them if, in the context of their relationship, he didn't break the rules. Unfortunately, we play with fire in the D/s world and it's easy for things to get blurry. But it's also important to remember that this world is also a world of illusion that we weave and sometimes that can obscure reality to the observer. I'm not defending this D/, but i don't have enough info to know for certain whether this was a case of abuse or whether this is merely something that looks sketchy or questionable but between the two adults involved was acceptable to them at the time. Also, it's important to remember that hindsight is always 20/20. i've consented to things i never should have but at the time i DID consent. my Domme at the time did things that i have since learned would be considered by many to be wrong and they ultimately were hurtful (emotionally, not physical harm) to me, but i consented. She had culpability for her actions but i also had responsibility. i could have revoked my consent, i could have objected, i could have used my safeword. i never did. So, although i do hold her accountable for the things she did irresponsibly, i also recognize that i consented and that what she did, however poorly, was not rape or kidnapping or anything like that. i can't look back on that, knowing now how badly she was handling things, and entirely condemn her for it because i gave consent at the time. i can't retroactively revoke my consent. i can say she's someone i would never play with again, i can say that i wouldn't recommend anyone else play with her. But she did not do anything to me against my will or consent.

Agreed - anything that crosses the line of mutual consent is no longer play, but has then become abuse. I hope she gets out of there safely, and I am glad to read that this was a FORMER partner of yours. That does not sound like the kind of person who should be doing BDSM play.

I agree with the others, it becomes abuse once it's no longer consensual.

It doesn't cross the line if there's consent.

What an odd question.

No amount of heaviness in play or level of protocol in power exchange is telling of abuse. All partners involved, as long as they are consenting to the situation, are not crossing lines of abuse.

Anything that is not consent to me is considered abuse. I am finding it hard to explain "negative intimidation" here... But some types of intimidation I am fine with, but intimidation that would frighten me to do something I am not comfortable with or literally frightens me to change my mind would be a form of abuse. In the case you mentioned, if my partner and I broke it off, then got back together, if he said it jokingly and I knew he didn't mean it mean it, but meant it more like "oh tonight we are going to play and really play hard" I would be ok with it. But if I felt that he actually meant it, I would leave his sorry ass.

I kind of think abuse is also a personal thing when it comes to BDSM and varies couple to couple. I know a man who was into punishing his wife for burning some parts of the dinner... to me that is abuse... to them? That was part of the fun of their relationship.

Anything that is not consented to.

It depends on the persons and what they agree to, anything that one does not consent to would be defined as abuse to me.

It sounds like she did not consent to this. If the purpose was to actually make her afraid, it sounds like abuse to me. S&M is supposed to be enjoyed, or at least enthusiastically consented to, by BOTH partners. Beating to make her actually scared is frightening, and, I would say, certainly abusive.

For me, it becomes abuse when it's no longer consensual. Subs say they like being controlled, but ultimately they have the most control in the relationship because they say are the ones who say "Yes, I'm going to allow you to control me". In one of my relationships, I was okay with him spanking me, pulling, my hair, and being generally mean in the bedroom because that's what I wanted and enjoyed. So, that was okay. However, he pinched my maliciously one day out of anger outside of the bedroom, and that relationship was over within a couple weeks. I think that the sub has to go with his or her gut feeling. If it feels wrong to you then it's abuse.