Not everyone with BPD cheats....but yes the "impulsivity" can include sex. But not with everyone.

Is she in treatment? If not...please help her get into therapy. For both your sakes. Try couples counseling if she refuses to go alone.

If you're concerned, you can always go get tested for STD's. But don't confront her about an affair - you have no idea if she really has done anything, and a big confrontation about it could send her off the deep end. Just be careful.