Say something. There’s no asshole here unless your friend gets pissy when you say “ah no sorry, I can’t.” In general, don’t do something that upsets you so many times you get worked up about it. Usually the person you’re doing it for will have no idea you’re starting to get pissed off and then when you blow your top, it’ll seem like it came outta nowhere. I’ve been on both sides of that and it sucks. Communicate.