I would not say anything critical about her or how she treats him. Criticism of her can be felt as criticism of him and/or make him defensive of her.

I would comment on his feelings. "You seem sad today." "I've noticed you've been down a lot this week". And then listen. Maybe he will talk about her, maybe he won't. If he doesn't, so be it. You could try again another time.

If he does talk about her, continue to avoid criticizing her or how she treats him. Keep your focus on how he feels. "It sounds like it really hurts when she says that." Don't offer solutions. If he explicitly asks what you think he should do, ask what the options are, and what are pros and cons of them.

If he starts to talk about her, that's great, but don't expect a solution and action right away. It usually takes time for people to get ready to change. Your unbiased listening will help him, whether or not he decides to make a change.

Don't be committed to him making a change; do be committed to listening to him if/when he chooses to speak.