The girl seem to have more than friendship feelings towards him. But we don't know for sure if he returns those feelings. He might just like the attention.
OP, you can try issuing him an ultimatum which may or may not work. But even if it works, he is going to get tired of your demands.
You've mentioned that when he was hanging out with this girl, you are usually at the office, working. Do you work alot and have not a lot of time to socialize with him? He might be hanging out with this girl because he is seeking for companionship and lacking it from you. With the added thrill of the taboo (of hanging out with a girl he isn't suppose to because she has feelings for him), he is getting pulled more and more towards her.
With further thinking, I think you should really divide more of your time towards spending time with him rather than for work.
Last edited by fearoflove; 29-05-13 at 12:22 AM.
When you give an ultimatum and your ultimatum is perfectly normal (and not just the expected whim of an entitlement princess/prince) to what two, monogamous people expect within a relationship and that ultimatum back fires on you, then you've just dodged a bullet and its a good thing that it backfired. Any man (or woman) that will keep an opposite sex friend over his SIGNIFICANT other does not value you or your relationship and its best it ends because of that ultimatum. There is NO "backfireing" if something comes to an ultimatum. It's win win whether you get your way or you don't.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion