I was'nt thinking about the conversion. Duh!! So yeah, he will have to replace both but since he's gonna replace the rear anyway, I'd go up 2 or 3. He could stay with a 15 front unless he really wants some unbelievable low end with 14T as well. I have always been told by my dealer and others that its best to go up in the rear as the chain can rub on the swingarm with the 14T. I have'nt heard any horror stories yet though. Its quite a bit more expensive, changing the rear but it also adds bling.
I got the 41T supersprox with gold DID chain. It looks sweet and performs great too!!