Having a potluck Iftar event is acceptable. In fact it is very much encouraged in Islam to share foods with others, what more during Iftar. However, the inclusion of non-halal foods in the Iftar could have been an oversight on the part of the event organisers. Appropriate advice should have been given earlier.
Iftar is not just another dinner. Iftar has its special meaning to Muslims, and there is decorum associated with that special meaning.