The best thing you can do for a homophobic person is help them connect with a gay person on a personal level. Actually, this is true of any bias or prejudice, introduce the person to a gay person who doesn't seem gay. Then after they have gotten to know each other and they can relate on a personal level, the bias and prejudice will usually go away.
Unfortunately, sometimes it is only against the one person that they have come to know. Often it takes getting to know more than one gay person for the bias to be removed.