Understanding autism

Hi, I’m just looking for some advice. My Sons girlfriend has autism with seizures and lives at home she is in her early 20’s. I don’t fully understand the situation but she is fairly capable in day to day life, stays at ours at the weekend and she communicates at a good level. However tonight she was distraught and was very upset on the phone because her family do not like my son and they keep having a go at her and my son over the relationship. On several occasions she has been threatened with being sent in to care and today they told her if she leaves she’s not coming back. She was also slapped by her mother. My son is very upset by the situation so has not helped himself by getting into arguments with the family. They believe it is my son who is the problem as they think he doesn’t treat her very well and he argues with them and also because it is ruining her routine which is the reason for her outbursts. The daughter says it is because they just keep picking on her all the time. On speaking with the mother this evening she said she only slapped her to stop a seizure and that speaking forcefully to her is the way she has been trained to deal with her, she isn’t allowed to talk as they say she is arguing and if she says anything they make her apologise with threats of care (I heard this myself). I guess I just need to know if this is how you are supposed to treat people with autism as I know a different approach needs to be taken. I really want to help by knowing a little bit more so we can give her some routine when she is here and support the family if this is the correct way. Thanks