Mtn bikes are supposed to be ridden. If your buddies are willing to lend you a bike, take it. It'll be nicer (and the ride will be better) than if you went and bought a hoopty. Just like borrowing tools: IF you find yourself going more than 3x you need to buy one.

Failing that, ebay and CL are your friends. IDK about your neck of the woods, but here in the DC area there's a boatload of newer old model bikes for pennies on the dollar on CL.

M