Freaking out

Youngest son and girlfriend just panicing me to death.  Supposed to go get her this morning to spend the day at our house and don't want her.  
Know she is crazy as a loon.   Saw some of their facebook conversation last night.   Not hacking his account anymore.  I gave that up about a week after he turned 18, although it was very hard.   He sits on the floor in front of me and types and I can see it.  Guess I should look away, but it is like passing an accident on the hwy. 
So she has had problems lying and accused a neighbor boy of abusing her.  Last year she accused her dad.   He says she has problems with hurting herself and is dangerous.  I have tried to have faith she is telling the truth.  Hard as I like the dad, but do know he drinks a lot.   The story started he hit her when drunk.  Guess it has changed now as son says he abused her for 11 years.
Well last night on facebook they are talking and I see her saying when she got back she would be "Rain".  She came back and was talking funny and pretty sure "rain" is supposed to be an alternate personality.  Later she told him to expect her to be "blank" for a few moments as she would have to control "Violence" to make sure she didn't attack him. 
He moved for a while and I didn't see anything, then moved back and she was asking if he thought she was lying again.  He told her he was going to have faith she would tell him the truth.   She asked if he was ok then he asked if she was.  She said she was sad and in some pain.  Said she did something to herself....He told her not to hurt herself or he would slap her when he saw her.
She said he should slap her.  Conversation went on about how he should slap her hard.  He protested, said he'd leave a mark, that someone would hear, and she just kept going on about how hard he should hit her "like when he was really mad".  She even suggested they walk to the park so he could do it.  He finally said he wasn't going to hit her as hard as he could.  She said "Ok.  Sigh".
OMG, this girl is insane.  She does hurt herself and wants my son to hurt her too.  I do not want him around her....But he is 18 so no power and he has already told me if I don't like her he will leave and I will never see him again.  I know trying to talk to him is worthless.  I will just drive him away.  But I am so scared for my baby.  He loves her so and he is so stubborn I know he will not let go.  He will think he can help her and she will destroy him.  And today I have to go pick her up and smile pretty and pretend I like her.  Thank God she hates kids and doesn't want any.  At least if he marries her, which he plans to do, I won't have to be terrified for my grandchildren.
I pray and pray that she will do something that drives him away or better yet find another guy and dump him, but then again I don't know that he would ever recover from that either.  I guess I need to pray the therapy she is in helps and she grows up (she is 16) and stops lying and acting crazy and can be a person I can truly like.