Free flight from Aegean due to disruptions a previous flight of yours in Business faced?
Anyway, your friend should probably book that ticket soon enough, I wouldn't imagine anyone would want to wait until the last days/weeks before their flight to book it, so perhaps you could risk it and wait a bit longer. Especially if it's that "free ticket" I mentioned, I don't imagine they'll immediately cancel it and since your tickets are in booked in either cabin my hunch says you're not in any immediate danger (again, depending on how far out that flight is).
But yeah, eventually you will need to take care of the matter.
I hope your friend books business class, somehow. Even if you cancel that light U fare completely and get nothing back from it, it's obviously a small amount for that booking class, plus you'll probably still get some of the taxes back.