Looking at the band plan next to me, HAMs have an allocation from 2300 to 2310, and from 2390 to 2450, at least some of which overlaps the RC band. In that area, anyone but a novice is authorized for all modes . . .
Amateur radio in the US does not have specific privileges on the 2.4 band we use for RC.
You also assume that he is in the 2.4 area where most RC occurs - I don't see any evidence of that . . . . This thing should have no more legal impact than if he were to put a Spektrum module in his Futaba, be it either on 2.4 or on a HAM band. He also states that the original RF section and antenna are still there and unmodified, once again, the same as if he were to run a 2.4G conversion module . . . .
I would be interested to know more about the system in the picture, though . . . brand, etc.
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Addendum: I just found the system in question - it is in the 450MHz band, in the HAM allocation . . . so no legal issues whatsoever if the end user has a HAM license . . .
Friends don't let friends become electrotarded . . . .