So perhaps the problem actually lies with her. I would say you can't do anything if she's said something like that to him already. In fact, acknowledging it might even make it worse for him.
If I had a partner who was making comparisons between me and their other partners, even if they were flattering, I would tell them that I didn't feel comfortable with how they were speaking. I think it's a very mono way to be, comparing current relationships to past relationships, telling your partner they're better than anyone you've ever been with, etc. (Not saying she's doing this, just in general.) You're really going to shoot yourself in the foot bringing this mindset into poly relations.
Ask her to rephrase her compliments to be about you, and you alone, not in comparison to anyone else. For example, instead of saying, "You make the best peanut butter sandwiches I've ever had" say something more like, "Your peanut butter sandwiches are amazing! I love how you spread the peanut butter all the way to the edges!"
Just something to think about. Food for thought, as it were.