I misread this. I thought it was about the person who felt the slight saying something, so I voted contact in private. If it's someone else
you're friends with, I think it's best to talk to the one maligned, but let them
deal with it because it's about them. You never know the history between these people and inserting yourself into it is not usually a good idea.
Now, if it's a public debate issue where it's in a discussion thread that they are posting these comments, for example, then you have more leeway, but still, I would contact the person I was friends with and ask if they were offended (that's assuming I caught it in the first place). If I thought the other was really out of line, I would contact them and ask if that's how they meant it. But, I try to let people fight their own battles.
If someone is really abusive on the forum, or other public space, of course, I would comment and if necessary defend the maligned. I have learned that things are not always as we perceive them online. Sometimes I have seen slights where they were not intended and also I have missed ones that were. I say be assertive and just ask them WTF?! Once you're clear that they are doing this deliberately (or they keep doing it passive aggressively), then you can ignore them, or toss a few barbs back and tell them to back off! Remember, on here, there are rules and you can flag posts, but you have to be sure they broke the rules first. It's always best to make sure it was intended (unless it's so obvious, such as calling you an f*ing moron or something without provocation).