I feel for you, dood. I'm falling for a girl who has a boyfriend too. It's a hard situation for EVERYBODY. At least she knows you love her, and you know she loves you.
The obvious solution, of course, is simply to kill her boyfriend:
You + Girl + Boyfriend =
You + Girl + Boyfriend
- Boyfriend = You + Girl =
If you go this route, be sure to hide the body really, really well. It would suck to have the cops drag you off to federal pound-me-in-the-ass prison on your otherwise happy wedding day.
OF course, you might have qualms about killing other human beings, in which case you're going to have to go for a more subtle approach.
decaffeinated puts on "serious" hat
How honestly have you talked to her about this? Does she
know you'd marry her if she wasn't with her current boyfriend? We tend to assume things that may or may not be true. If you're as good of friends as you say you are, I think you need to march right up to her and tell her EXACTLY what you want to say. Tell her you love her more than anything, and if she'll give you a chance you'll make her the happiest girl in the world. Tell her you'd never leave her, or hurt her, or cheat on her. TELL HER.
Face it, the only way you two can be together is if she dumps his sorry ass and gets together with you. I'll admit, that's unlikely if she's planning on
marrying the dude, but SHE'S NEVER GOING TO if she doesn't know exactly how you feel. Vague "I love you"s don't count for much. We're talking specifics. And, hey, she probably isn't going to call him up and tell him to go f*** off and then beg to have your babies, but very worst case scenario, you'll at least know you didn't leave anything unsaid.
And don't worry about ruining a good friendship. Once she marries him, her friendship with you is over, period. If you're ever going to have a chance at being with her, NOW is the time to act.
Just my two cents, dood. And good luck!