Are you even gay, OP? B/c your comments make you sound completely clueless about the process of coming out. It does NOT matter whether we're talking high school or college. Your niece and her stupid friends have no business ganging up on some guy who they think is gay.
If the guy is gay, I assure you that he's already dealing with the gay issue internally or by talking to other GAY men. He just has no interest in telling acquaintances, and that's what your niece is to him, some chick he talks to periodically but has no real deep connection to, that he's gay.
He has his own reasons for not 'coming out' to these busybodies, such as: a) they gossip and he's not ready for everyone to know, b) they are acquaintances and he has no interest in discussing something he's processing with them, c) he's still coming out to himself and needs time to figure it out before he outs himself, d) he's expecting to get a deal in the NFL and he's TERRIFIED about being outed before he can establish himself, e) as you said his parents are homophobes, he depends on them for financial and emotional support, f) he's not gay but he's extremely guarded about sexuality possibly b/c of abuse or something else from his past, g, h, i to z) lots of other reason he has that leads him not to come out to this particular group of insensitive idiots who are bullying someone by doing an 'intervention' (b/c seriously is he an addict that's impacted their lives by his behavior?).
Clearly, your niece was raised by wolves who didn't teach her any damn values or boundaries. Something she'll need in life for her own success and psychological well being.
Oh and they aren't actually gay positive friends. Therefore, they don't need any support from us. They are as disgusting to some of us as homophobes are b/c instead of treating their 'friend' like they would any other, meaning, maintain boundaries and allow their friend to deal with something that doesn't impact their lives on his own, they've decided that they are going to bully him, b/c that's what they are doing, bullying him to come out for for their own satisfaction.
Underneath their behavior lies the behavior of someone being passive aggressive and unable to empathize with others. So no, they ain't much better than homophobes who are at least upfront with their disgust and hatred. Instead these girls are more insidious since they are actually doing something hurtful and insensitive against someone they claim to care for as a friend. Don't see love or concern, only smug satisfaction of a bunch of undercover mean girls.