That's cheesy indeed. As far as I would say, stay a friend. Really. You love her, you both had great moments and all... but if the other guy is a friend right ? So I'd say let him enjoy too. It's not his fault if you didn't make a next step with her.
But okay if for example the guy were clearly an [anal opening] or disrespecting her, you could make her open her eyes on the situation (in a friendly way, i.e. don't take winning for granted, she's the decision taker).
But if they get along, I'm sorry but you had your time. The only thing you could do is be there for her when at bad times. Again, don't necessarily give your opinion on your advantage, that's just selfish. When at moments you're thinking like "why not me ??", just don't say it. Respect her, comment if necessary, and wait...
Good luck anyway.
SK.