Leave him alone. Its his baby.
Just like classic cars and boats logic doesn't come into.
If the additions provide an increase in enjoyment and experience of flying the machine and are considered "worth it" to the owner I see no problem investing even if its not going to recuperate the value. He can always transfer them to another plane later if he gets another one. This mode S stuff and 8.33 won't go away.
If it was a buisness buy then I would take a different few but to a private owner its not a problem.
Hell I have a 350 quid hand crafted model of an aircraft I used to fly sitting in my house. Value on ebay 20 quid. For me worth every penny.