Longterm boyfriend's ex added me on Facebook

My boyfriend's "first love" (as she refers herself as) has a history of severe BPD. She was convicted of arson in the last three years, she accused my boyfriend of beating her in high school (complete with self-inflicted injury by banging her head on the countertop), and she is only allowed to have a relationship because her therapist allows her to.

Anyway, in the last few years she's been on this big self-improvement kick. Kind of a "live laugh love" type. I met her once when she was bartending and she was mostly normal. I got the friend request a few days ago and I'm just kind of.. curious? Is she trying to be amicable and is she genuinely happy for us? Or does she have some crazy lurking under the surface? I know you can't really treat BPD but I was also diagnosed with it as a kid and don't really show any symptoms as an adult.

I get along really well with my ex and I figure the fewer enemies, the better, but I'm not sure that I believe she's turned a new leaf.