Let's say that another person rented a complex aircraft from the FBO you rent from. It had a mechanical issue and is at an airport 3-5 hours away. That pilot left the plane and found another way home. The FBO got the plane fixed but it is still 3-5 hours away. The CFI asks if you want to fly up there with him and fly the plane back. All of this is obviously for free. Can you as a private pilot accept? Would the free flight time be compensation? I'd like to say this is a hypothetical, but it was a real question a few months after getting my PP.