Hmmm, that's a really good question.
On the one hand, if you and he have reached an agreement regarding time, terms, priorities, etc, does it really matter what anyone else thinks or says?
But on the other hand, if you suspect that he's not being completely honest with her, is it possible he's also not being completely honest with you? Are you both being told what you want to hear?
If it were me, I think I'd want a sit-down with all three of you together to make sure everyone's on the same page to try to prevent drama later. If that's not something he's on board with, then I'd be wary. In some ways it may not be any of your business, but none of us live in a vacuum. His other relationships can affect you and your relationship with him. I don't know that your friendship with your metamour is even the biggest concern right now- I'd be more concerned with the question of "Is he managing his relationships in an honest and healthy way?"
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack