I need more information, I've had similiar experiences with this. But I'm going to need more details. 
Now this straight guy friend. Is he teasing your gay friend, playing with his emotions? If so- instead of fighting amongst each other, I would team up and call the straight guy out on his shit. Believe me, straight guys aren't so tough - you can handle it. 
Or is this so-called straight guy not even really straight at all but another closet case, and more drama unfolds that way? Is he one of those guys that can't deal with a gay label, is to chicken shit to hold hands with a guy in public- but LOVES mansex nonetheless? I know PLENTY of 'straight guys' like that myself. They do also claim to love women, but do I ever see it? NO. 
Look, there has to be SOMETHING the straight guy is doing to making him go head over heels for him. He might be protecting him - many gay guys like to be protected a bit too much instead of having to do the protecting. 
Your gay friend is being an idiot, but it sounds like the straight guy is being a typical straight male predator and trying to go after a perceived weakness. I would do the same thing. I would totally give the cold shoulder to the nice gay boy that was flirting with me, and I went with this mean straight boy that only broke my heart. It's very immature, but some gay guys do grow up, I promise!
What happens is- and this is the only way gay boys learn it seems: He's going to actually need to have SEX with the straight guy to get over his attractions.  I'm serious. It will never happen, but the thought that it might happen is too appetizing for him- so make it happen! It's the ONLY thing that will work. Some how, you're gonna have to get those two naked and prove that he's not gonna be able to please him - because he's straight! Maybe then will he get some self-confidence. 
Maybe you and the straight guy (assuming he's an innocent and likeable standby and not somebody that is manipulating here) can also team up to teach him a lesson. Oh God, I am so for calling people out on their shit. He needs to have a hard wake up call. Funnily enough, as much as we crush after straight guys- when we actually have sex with them - we see just how straight they are, and it's not sexy anymore. 
The straight guy is naturally more appealing to him now, because there is more mystery. You're trying too hard, and that is turning him off. If you really, really want him - then try to be mysterious like the 'straight guy' but he really doesn't sound like he's worth it to be honest! 
I'm available right now. So hey. 
