You borrow someone's shit and fuck it up, you replace it. That's life. If you're not willing to do that, don't borrow other people's shit.
It'd be one thing if it was a board with some mileage on it and a small core shot. Then, sure, just fix it and it's all good. But, IMO if you do that type of damage to your buddy's week old board, you owe him a new one. That's what I'd do. If simply doing the right thing makes me "mad", then I guess society really is as shitty as I assume it to be.