It could also be that he's telling the truth, in that he does have a hard time with her being poly... even though he is as well (or is he a swinger?). It happens, too. Maybe you could try and clarify it with him?
I think honesty is pretty important, but I'd always ask the person concerned first, as otherwise it's more like talking behind each other's back.
At any rate, I wouldn't be confortable hiding my feelings, so you should definitely talk about it with either of the two of them, or even both.
Me: 30F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon