Something similar happened to me recenrly. Bought a family saloon from a friend of a friend. He said the cam belt had been done at 50k, but had no record/proof. I said that I'd therefore assume it actually hadn't been done, and do it ASAP. 2nd day after purchase, the cam belt fails, and I have to shell out £1500 for a new motor.
I told the seller what had happened, just to keep him informed. I never had any thought to ask for my money back/a contribution.
After all, as soon as you sign the document, it's your car, right?
On the subject of mates, I think I'm secure enough with mine to tell them they're being unreasonable. He's being a d*ck for asking for money, and the OP is being a bit wet for taking that kind of sh*t.