OP, I also met my DD's BF's mom. Basically we both wanted to make sure we are on the same page regarding curfews and expectations. I think she also wanted to let me know she thought they were close to getting intimate, which I knew, and told her I'd had the safe sex talk with DD. It turned out, maybe unsurprisingly, that they were telling each of us some half-truths about what the other family allows to get more freedoms.
Honestly, I don't know how it would have gone had we not been on the same page, or if either of us disapproved of the relationship. As it was, it was clear that both sets of parents realize sex will happen if the kids want it (I know it has, pretty sure she is not yet aware of it), but if so, better that it be in their homes rather than outside somewhere, and that it be safe.
My own opinion is that our kids are too young for sex (15) - no legal issues here, just emotional maturity - but knowing my DD, apart from trying to break them apart, it was going to happen (and it hasn't been all smooth sailing, as I've posted about). Other than the sex, I am happy that my daughter has a nice, devoted boyfriend and that they are clearly in love and learning to be in a relationship.
I see your worry about your DS's GF's neediness and perhaps mental issues. All you can do is keep the communication open. I ask my daughter to let me know where she is and with whom, and she has a curfew weekdays and on weekends (later). I do ask how things are going with BF, from her point of view I probably do pry at times. The kids were not all that happy his mom and I exchanged some information but in the end I think it's for the best to know what the expectations and boundaries of both families are. At this age, you are, in a limited sense, coparenting a young couple. Perhaps it's also cultural or because of the fact that DD spends a huge portion of her time with the BF in person or online, but I've felt a lot better since having spoken to his mom.