I'm not sure a commercial cert is required,
I am absolutely, 100%, completely sure that Grant has to pay the direct costs -- otherwise, Grant is clearly providing pilot services (ferrying the airplane) in return for compensation (free flight time).
as the two people clearly have a pre-existing relationship. As long as Grant doesn't log the time for use toward a new rating, he hasn't received any compensation, and isn't operating for hire.
If anything, a pre-existing business relationship would
hurt, not help, Grant's case, and with a pre-existing relationship, not logging the time would make no difference. See
Administrator v. Murray:
"Nevertheless, compensation need not be direct nor in the form of money. Goodwill is a form of prohibited compensation. Administrator v. Blackburn, 4 NTSB 409 (1982). The evidence establishes that, not only was respondent a friend of Mr. Fitzgerald, but he had done work for him in the past. Interpreting the facts in a way most favorable to respondent and assuming that he really had no expectation of any kind of benefit, strains credulity."
Grant flies up solo (no passengers, no compensation, no violation of 61.113),
It is that solo leg where things get tricky -- pilot service is being provided, and there are several ways that the FAA could find compensation being provided if Grant doesn't pay for that flight time.
and the owner flies back, assuming all costs.
No problem there -- Grant can accept a free ride home.
Now, if the owner isn't qualified to fly the airplane (no medical, etc), then there's an issue on the return flight, as the owner is a passenger, and Grant must pay at least the pro rata share.
No good -- no "common purpose" for the flight. The FAA's interpretation of 61.113 says Grant must pay all direct costs for all the time he's flying to be FAA-legal.
This would look much more like a "hired" pilot who would need a commercial certificate.
As I understand the FAA's perspective, to them, it
all looks like a "hired pilot who would need a commercial certificate." Remember that the FAA Counsel has always said 61.113 will always be interpreted in the strictest fashion. Caveat Aviator!
Look, we've all done stuff like this in our Private Pilot days. The FAA doesn't prosecute because nobody complained. In Grant's case, if the owner doesn't say anything, and Grant doesn't say anything, the FAA will never hear about it and the flight will disappear into the dusts of aviation history. However, if anything bad happens, or if for some reason Grant's friend becomes disenchanted with Grant, or of Grant's friend first asked the local FBO for one of their pilots and then balked at the cost, the FAA may well get involved and Grant will be the apple-mouthed guest of honor at the local FSDO's next pig roast. If Grant's willing to take the risk, that's Grant's choice, but if I were him, I'd want to know about that risk, and while it is small, there is definitely legal risk here unless Grant pays the owner for his flying time.