Last weekend on a mountain bike ride I traded bikes with my friend so I can test out a bigger suspension bike. He was happy to ride my cross country bike because we were riding up some grueling hills.

After the ride, we noticed that the seatpost on my friend's Specialized Enduro that I was riding had slid down and hit the rear shock. We didn't think much of it, but friday when we went on another ride his shock completely failed. He took it to the shop, and they said it was from the seatpost hitting it, and that he has to either get a new one or get it fully re-built.

I'm aware that the seatpost issue on the Enduros is a design flaw, and this year they have tried to fix it. Also, the shock was having some issues leaking air and squeaking before I ever rode it. So my question is:

Is it my responsibility to replace the shock for him, since I was riding it when the seatpost hit the shock? What I have done is looked on e-bay, and found some similar shocks for good deals, and told him I feel like I should either split the cost or pitch in $100. Is this enough or too generous considering the details?