1> If you friends with her BF then he might get pisses
2> if things go wrong then youre friendship will be damaged if not ruined
3> More likely than not she'll stop telling you EVERYTHING because you wont be her best friend, in a sence yes but who will she turn to when if she's upset or something with you? She might keep it in and that makes things worse?

Depends really, let their relationship be and if it finishes give it a few weeks/months if you start getting really close go for it