Planned a special celebration meal out for six. One of them got back to me asking if invite included X - it didn't, but no problems so said X was more than welcome.
It's just hit me X has a severe fish allergy, I think it's more ingesting it though (but there was an instance where a chef had fish oil/meat on apron). However, at least two/three of the original six are likely to want fish - two really like it (and one of them doesn't like pizza or pasta), the third has to restrict carbs for health reasons and doesn't eat meat, so can't have pasta/pizza/risotto/meat options and there aren't any veggie salad options - that'd leave them with no choice.
Absolutely not changing the venue (three would probably want fish anywhere they went anyway), it's a surprise celebration meal the recipient loves and means the main four they'd want can walk.
What would you say, do? It sounds selfish going back and saying we're eating fish, but even worse to un-invite.