Sorry you said physical abuse in title.
I'be not been through any of this so feel free to ignore me but...
Professional counsellor or other independent professional to see what, if any, abuse is on going.
Bruises - do they appear after play with child at school or after seeing dad? Your friends needs to ensure play at school does not result in physical bruises/injuries and when it dorsbhappen it is documented by school and her.
"When she is out he messages them through her..."??? Your friend's children are being messages by him? How? Why is she passing on messages from him?
She sounds like she and they are still stick on abusive relationship.
She needs to call women's aid.
They should not 've using their former home (now her home??) for him to access her and kids if his behaviour is abusive.
If there is a fear he is abusing kids she needs to get help to stop it.
It really sounds like emotionally she is still mixed up with him and he thinks he is in charge!