Just let her be. Don't worry until she starts making major decisions like living together or getting knocked up. Any unsolicited advice at this point is just going to shove her deeper into whatever she's doing. Just keep an out for her and when things start to get really out of control, then you might want to step up and say something.

If she's young, it's not all that atypical of a behavior. Serial monogamists loathe not being in a relationship with someone and are sometimes quick to move into the next. If she was seeing him behind her ex's back, so be it. No use in trying to fix the past when it comes to that and it's between the two of them, and no one else. less

My good friend just got out of a pretty shitty relationship. We were all proud of her, but then that turned into a state of shock when she let everyone know she was seeing someone else already and they were "Madly in-love"
Now there's no exaggerating here, it was less then five hours. So a bunch of red flags got thrown up.
Was she seeing this guy the whole time behind her ex's back? Is she nuts? Was she so emotionally abused she just latched on to the first nice guy that walked into her life? What's her deal?
I normally don't get involved, but shes like a sister to me. Should I say something? I don't want to shit all over her happiness, but I don't want to see her jump head first into something that could fizzle out and turn bad for her either.
Any advice would help. Thanks less