This was a bit of a red flag for me as it seems BPD mothers like bad guys and want their children trapped and vulnerable with the wrong people.
I'm pretty sure that not all Moms with BPD would be that nefarious
Maybe there's just something about your boyfriend she likes; he doesn't "rub her the wrong way" or maybe the fact that he isn't her child gives her a different attitude and opinion of him? If he makes you happy and you love him and value him, I think you are "safe"
In any event, I think BPD mothers will do the whole black/white thinking with just about anybody. She's gonna love this guy for some period of time, and then turn on him, and it could go back and forth, until she fears that she's losing your attention (and/or his) and will paint him black.
I'm also not sure that you could assume that about every BPD Mom, either. Sometimes our BPD loved ones (even Moms) have great relationships with everyone else, but tend to struggle with their own flesh and blood family members moreso. She may have a good relationship with this man forever, actually... .
We just don't know, and assuming that things will go south for sure could be very detrimental to the relationship--expecting the worst to happen colors our interpretations and reactions to events (and relationships) and isn't helpful in the long run. I'd have an open mind about her motives here, and try to feel positive about the situation... .
I think it's a blessing that she likes your guy, zubizou87