/cgl/ - Cosplay & EGL

>con coming up soon
>going with three friends
>someone new cosplaying the same fandom asks to share a room with us
>we don't know him well but agree
>just found out that he's been shit talking our costumes behind our backs in a discord and sending hypercritical messages to another cosplayer's tumblr
>his costume is a piece of shit he bought off aliexpress
I'm mad about this and wish we could kick him out of the room but he's already paid and my friends think we should give him a chance. I'm not good at hiding when I'm pissed off though, he's going to notice that something's wrong. How can I tell him that he's being an asshole and should stop stirring the pot without making the situation worse?