Just my opinion, but this should be a pretty happy time in your life with your wedding being so close and all. As such, you have every right not to deal with other people's BS and be absorbed in their problems. I'm pretty sure that your mind is working double time trying to tie up loose ends for your big day. So my advice is to just ignore them and enjoy your special time.
I'm also sure that this whole thing is not going to blow over in 17 days so you can be a giving friend at that time. As for getting involved. Take my advice STAY OUT OF IT!!! Somehow something will end up being your fault. You also never know with life....your BFF can end up with the guy she is having a fling with. You saying he is this and that can backfire on you down the road....
Again just my opinion......