Two things here, no three.
1. Crazy is going to crazy. Set boundaries and put them in their place.
2. Lock down your profile if you can to make it private and just between friends, or if you want to keep it public know that you're in for a fight.
3. The whole "love on the dl" thing. I don't know who you're talking to or who is talking there exactly, but in the end you and him have to be willing to be public with everything and run her off if need be. Actually, it is his mom, and he needs to put and end to it himself. He needs to stand up to her and enforce consequences for her actions. At some point in all of this he has to be willing to do whatever it takes to keep crazy away, and if he's not or he ends up trying to hide his relationship with you or something to pacify mommy, dude's got mommy issues and needs to grow up before he's ready for a relationship.