Everything I'm reading seems to suggest that JJ Gabriel is a sound lad and gets on great with everyone at the club, but his Dad's just an absolute lunatic, who has already burned bridges at Chelsea, Arsenal and West Ham academies before moving his son up north. It's a really sad situation for such a talented young lad, but his Dad's just out to exploit his talent.
I can't see any way you could attribute any blame to United, or expect the club to have done any more than they have for the player.