1. fix it, 2. Part of a complete repair is estimating how long the equipment should last. A few months later, the machine develops another fault due to a bad belt. I replace it for free. I assume in that case that the belt just wasn't up to snuff when I installed it.
New parts can fail.
As I said, you get the benefit of the doubt. Perhaps there is something in the machine that overloaded the belt and made it fail. I've seen that a few times. That's what my warranty is really for. I'm in business to make money, but not if I have to make enemies in the process.