What should I do?

Tread carefully it may be best to just say Hey man I am here for you but only you can decide what's best for you and put it in his court.

Why does he want you to tell both of them your opinion? That’s like putting you in a spot. Tell him you’ll give HIM your opinion and he can do what he wants with it. But to tell his girlfriend about how you feel about their situation is only going to make her feel like you’re ganging up on her and piss her off even more. So I’d avoid that situation.

It's a no-win. Tell them you have no opinion - it's their business.

Tell them you don’t want to get in the middle of it.

Stay out of it.

The way this works is - if you agree with him, there's a good chance she will never like you again. If you agree with her, one or both of them will wonder if it's because you've got your eye on her. If you say she's a rotten witch, in a day or two when he likes her again, he'll remember you think she's a rotten witch. Once you start to play this game, you lose.

Stay out of it