Bfs fwb ex invited him to her gathering?

My boyfriend and I have been together since March of this year. Before i met him he told me he was casually dating this woman but it wasn’t really going anywhere. So he basically told me he felt like they were better friends/she was treating him like a friend so it turned into a FWB. So then i met him we dated a month and then became a couple.

My boyfriend told me she verbally invited him to her housewarming and he was going. I told him i don’t think it’s appropriate for him to go but he said “well I’m not staying , but I’m going to go”

TL:DR; i am slightly upset because , i don’t think he should go for the simple fact that they were together whether official or not and he insists on going. I feel like I’m overreacting but , she’s considered an ex in my book and i don’t know how close they are for her to invite him