The NCAA is very messed up. There is no reason that this should be a problem...it is just a friend giving a friend a couple of tickets. I certainly don't know all of the NCAA rules (I doubt anyone does since they have so many), but unless there is one that specifically says that a scholarship athlete cannot accept complimentary tickets under any circumstances then nothing was done wrong here.
The NCAA, I believe, is trying to limit undue influence on kids. What influence would Carmelo be trying to exert on Mayo? To come to the Nuggets? Please! The draft will decide where Mayo goes and the NCAA doesn't govern the NBA anyway. It would be different if Mayo was still pondering his college options and a high profile person like Carmelo from Syracuse was giving him things...but that is not the case here.