What usually happens is that when you tell a friend something like that, even if you're right, he automatically goes into defensive mode and blames you for not supporting him. A tactic that works better is to invite him to stuff without her in a natural way, without making a big deal out of it - a beer, a meal, whatever - and just be there when he needs you. If at some point he directly asks what you think, that's when you can be honest but gentle, like "I feel like she controls you guys quite a bit" instead of dropping a full-on criticism. In the meantime, don't disappear yourself, because that's probably what he's feeling from his circle while he's in that relationship, and that's what hurts the most.