To play mind-reader for a second (always dangerous I know),
I think she's feeling like the excuse was a way to hide her or avoid acknowledging their relationship, even to downplay her importance. Being swept under the rug inappropriately would get under my skin.
My partner Audrey and I have discussed the fact that it's important to me that her suitors know that I'm in the picture at "the appropriate" time. Defining "appropriate" took some trial, error, thought, and experience. Some around here feel like metamours can be totally hidden from one another while keeping good conscience. I personally disagree.
In this case, I don't think that's what the husband was doing. Several of us have given the bloke the benefit of the doubt by guessing that he did it to deflect any ill will from the wife. I still think that seems plausible...but how can we ever know why he said what he said?
It still seems like scheduling (with grace from both partners as well as flexibility) is the best prescription, along with communication about what each of their priorities are.
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.