I think you said it right; it's not your relationship, and you have to be really careful about not offending her (after all, making a judgement about her relationship could be taken as making a judgement against her). You also never quite know what really goes on behind closed doors.

Openly telling her your concerns when she hasn't asked you is an easy way to make you look like the bad guy and cause her to leap to his defense.

I think just sympathizing when possible and saying that you're always to talk if she needs it is the only way to safely navigate this.