It's a reasonable boundary to have. If I were you, I'd not encourage him to hang out with that friend but I'd also not try to prevent him from it. 'Sure' instead of 'yes, please do go out with her'.

However, personally, if my partner expressed a wish that I join them/interact with a person I've wanted to avoid, I would do it, because they're my partner, I love them and one potentially uncomfortable afternoon/evening/night is a tiny sacrifice to make. If I proceeded to dislike the person after that, my partner should respect that.

Dying on hills of principles, especially when they're so low stakes, is never a good sign in a relationship.