Sully is the hero, but Mike is the buddy. Mike is only loyal to Sully, but he is magnificently loyal. Mike sees Boo as the danger she really is, not just a danger to their well being but a danger to their relationship.
Note that Celia is Mike's love interest, but she is no danger to the most important thing in Mike's life-- his relationship with Sully. He gains all his significance from that relationship, as well as the only real moments. Mike isn't just a satellite of Sully's, he's his coach, his advisor, and, of course, his best friend.
What Mike sees is that Sully truly loves Boo, and that is a real danger. More dangerous than a girlfriend, more like a wife. Or a daughter. No one can compete with that, and Mike recognizes that once that relationship is solidified, then Sully would give up everything-- his career, his reputation and even his relationship with Mike-- just to help Boo. Mike sees the love for what it really is, and that puts Mike's whole life in danger. Boo is the greatest threat to his well being.