I can't help those who don't want to be helped

Your friends fears are likely based around what he thinks might happen to his mother should he leave.
There's no easy way to do this.
He has to get out of there !!!
Ask him if he has relatives he might stay with for a while.
Nothing is likely to change by suggestion alone. Unfortunately !
Which means at some point there's bound to be some form of physical intervention.
Either dad being removed; arrested and charged with physical abuse or assault !
Mum too ! Not intervening when you know abuse is occurring makes her guilty too!!
Or your friend being removed; put in a place of safety and an order restricting dad not to come within a certain distance !
But the longer this continues, the more pain your friend will have to endure !
Until one awful day.......

It's common sadly. Listen instead and be a support to him. Remind him that the shit they tell him/do to him isn't true. That he is better than this. And hope he hears you. I know this is really tough. But he has to figure it out.

Your friend probably fears retaliation from his dad. I think the best thing to do is maybe mention it to a school counselor - maybe that way you can get them to pull your friend aside & have them do what they can to get him out of that situation.