Unfortunately this is a really tough situation I'm sure you know that, obviously this has to be done no question about that but I do believe the best way to go about this is to try have some damage control. Having friends that are also her friends is always really hard when it comes to a break up let alone dealing with the fact that your dumping her cause you realize you are gay. Everyone is going to view it differently and the best way I think you can minimize the damage is to hope that she is accepting of you coming out. Hell she might even be willing to just tell people you guys just didn't think it would work out without releasing that part of you.