Because if a buddy says to me, "Hey, wanna come to the game with me?", unless he says, "btw tickets at $80" or whatever," then I'm assuming it's on him.

Now, I think the friend should've said to Birthday Guy, "Cool, would love to come! How much are the tickets?" Would've cleared things up from the jump. So definite communication issues between the boys here...

... but if I get invited to a thing and they don't tell me "tickets cost X" beforehand? I assume I'm along for the ride and I'll just get my guy dinner or drinks later. So if we *had* to assign someone with making the first mistake, I'd say it's Birthday Guy for not establishing ahead of time that this was a ticket he was looking for someone to buy off him.

But I think it's subpar communication all around-- and unless the tickets were insanely expensive, if you're in a friendship where you're going to games and you're buying dinners/drinks for each other, I don't think it's worth getting angry about tbh.