How do I help a friend sort his life out

How do I help a mate. Can’t break up with his gf

Not that he hasn’t tried. She cheated on him. He cheated on her. In fact he’s inlove with the women he’s cheating on her with.
He’s told his gf how unhappy he is. She storms out and comes bk with ideas trying to make it work. Then he had to move towns for work. They none stop fight. He tried to tell her to stay and it was over. She breaks down threatens to hurt her self. Says she can’t afford to live on her own. So last week she moved with him. Away from all her family with a man who doesn’t want to be with her. All they have done is argue since they moved together. He’s a coward and weak but do you think they could ever work this out. I think he’s under the delusion. They can sort it.

Apart from work he’s not allowed out of her sight she follows him every were. Checks his phone the whole thing is a joke. She checks his underwear and phone when he comes home.

The women who he is inlove with is perfect and he’s ruined his chances of happiness. He said he’s waiting for his gf just to get sick and move out but I actually don’t think it will happen.

What help advice could I give him. He’s almost in an abusive relationship it’s very toxic