If a commercial pilot wanted to provide pilot services for free to fly a plane that is being rented entirely by a friend that wants flown out of town, is that legal?
What a number of people have at least suggested, in there hypothetical situation posted, the past I put in bold is the real question.
If indeed the non-pilot friend is the one renting the airplane, nothing wrong with the situation, even if the pilot is paid big bucks.
But "renting the plane" in this context means more than "paying for the plane." As others said, it involves something the FAA calls "operative control." I'm simplifying, but In this context, one thing it means is that, the lessor has given the renter friends has full responsibility for the airplane, including any damage that might issue to it, and the right to select anyone as a pilotwho meets some objective qualifications,, not just from a select group okayed by lessor.
I've seen situations like that, always involving a relatively complex "dry lease" contract, but never one involving an FBO rental.
BTW, "share the cost" is in the title of the thread but is pretty much irrelevant to the discussion. The scenario sounds like a trip for the benefit of the friend, not a "common purpose" flight. so even cost sharing would be improper.