This could be a good arrangement for both of you, just make sure you talk about everything up front and draft a rental agreement. (As my lawyer once told me, an oral agreement isn't worth the paper it isn't written on in the event you have to enforce that agreement.) If you lock up the wheels on landing and blow a tire, then you pay for it -- that's fairly simple. But there are some more complex situations you could face. For example, what happens if you're flying it and a valve breaks? Now you're probably looking at a top overhaul for something that is likely due to accumulated wear and tear and not directly attributable to the time you were flying. What will you and the owner do then? If it was a FBO' plane, you'd just hand them the keys and write up a gripe. If you had it in partnership, you'd both be in for a fair share of the maintenance. Would your buddy expect you to return the airplane in the same condition as when the flight started? Or what it you lock up a tire for a few seconds but don't blow it and just flat spot the tire? You've taken lots of useful life off the tire, but it still has some remaining. What then? Would he expect you to buy a new one? Would you expect him to leave the old one on because it's still good?
This is why mini said its a good way to ruin a friendship. If you both have the same expectations (read: written, signed, notarized rental agreement) then you'll save your friendship,
If you're thinking about flying this plane a bunch, you may consider asking him to make you a partner, even if just a limited partnership. (I think the legal term is subordinated position, meaning you have no say in decisions about the airplane, since you don't own the majority of it. If he wants to paint it pink and move it to Florida, you're along for the ride unless your agreement stipulates how the owner must make it available to you.)
For example, you could be a limited partner for 12 months for $2400, payable in monthly installments but due every month even if you don't fly. (I'm just picking numbers out of the air for the sake of example). Then you can fly the airplane for up to 100 hrs for the price of gas and oil you use, plus 50% of annual insurance. At the end of the year you walk away free and clear. You'll lower the overall insurance you both pay (assuming you have similar or better flight time and ratings to the current owner) as you can normally have about three people listed on a policy for no additional charge. You'll save him money by contributing to the indirect expenses through your limited buy-in, because the more a plane flies the cheaper it is per hour. You both except some risk and both stand to gain. Somewhere there's probably a set of numbers/conditions that will make you both happy.
FWIW, when I owned a Grumman Yankee I had a lot of friends offer to rent the plane from me. I was never interested in having them ham-fist my plane and I didn't need help sharing the costs, so I always told them it would drive my insurance up to the point where it wouldn't be cheaper than renting from the FBO since they didn't have any time in type. That was true, but even if it wasn't, I just wasn't interested in sharing my mistress with my friends. No one takes care of your things like you do. That's just a fact I didn't need to prove again. You may have more luck with your buddy if you approach it from buying into the plane (thus you will treat it like a plane you own, instead of one you rent). Find out why he's interested in renting it to you (he likes you and is one helluva guy an wants to help you out, he can't afford it on his own, he doesn't fly it as much as he used to, etc), and you can better craft an agreement that will suit you both.