[Aniki]

After reading all that, I'm surprised that everyone is encouraging the guy.
In my opinion if you're in really loved with her, that doesn't give you the right to hit on her.
You think the best way is to tell her what you feel. And what if she says she doesn't feel the same for you, then what? Do you really think everything will go back the way it was, that you two will just be office-mates, friends?
Instead of dreaming all the time you should snap out of it and start thinking with your head. You say that she belongs to another, but don't know if she loves him. Don't rush, try to find out more about her relationship, does she really love him or maybe she's with him because she doesn't want to be alone. If that's the reason, then and only then you should make your move.