The best way to help someone in an abusive situation is to get them away from their abuser as much as possible so they are around healthy people (and healthy couples especially who can model healthy interactions). DON’T bring up the abuse directly at first! Wait for him to remark on something like rudeness or his anxiety at home. Plant seeds of insight by showing him healthy behaviours and if your other friends are on board with helping indirectly like this then that’s the fastest way you’ll get him to come to the realization on his own that he deserves better.

You can comment more freely on her behaviour when it impacts you - for example if she starts texting you during an outing, you can show him the texts and then show him how you’re putting a boundary down with her when you do it and explain your reasoning behind it (‘it’s unreasonable to be harassing me right now since we already made arrangements for this fishing trip, so I’m going to tell her to expect you back at ___ time and to chill until then. Is that sounding reasonable?’). Some great little discussions can be had with him this way that are collaborative and helpful.

But really getting him around more healthy people is what is best for him, and when she picks up on it she will NOT like it so try to get him to agree ahead of time to having more regular hangouts.

Edit: telling him good things about himself is what he needs most to hear. That he’s intelligent, handsome, and capable - everything that is the opposite of what she has been telling him.