I like your idea, I also think 4% is pretty fair. However, letting him keep the altcoins is not fair because who knows how much they worth in the future. I suggest that 4% for him and he needs to exchange the NMC to BTC for now and save the other altcoins in his account and them out in the future.
The value of the altcoins compared to BTC can go down as well as up.
If the altcoins were to be shared amongst the group then they would have to be converted to BTC before being shared out. Would you care to specify what exchange should be used and what exchange rate we should accept? What should he do if there is what looks like a temporary blip and the exchange rate for the coins drops? Should he hold onto them indefinitely until the rate goes back up?