<human>: Can you condense the text into a few key points?

We've been together for one year and we're very serious. All the following information I know from various stories she has told me.

She hangs out a lot with this one guy from work, who's known to be a bit of a womanizer. They spend their breaks together, go to lunch together(don't know how often it's just them or they're with other people as well), he gives her rides home. At one point he said he thinks they're really really alike and that she should add him on facebook so they can communicate more - both me and my girlfriend agree this was a bit too much and borders romantic interest.

He is in a relationship as well, actually he's engaged, and I know his fiancee is jealous of their friendship as well, as she doesn't let him contact my gf outside of work(I suspect he cheated on her before).

I am jealous because of this situation, their friendship grew and grew and I became more uneasy about it. I am convinced my girlfriend doesn't want to cheat on me, but who knows in the future when they're away to a conference(which will happen in a few months) when we'll have some trouble in our relationship - he will be there, always.

I planned to just let these feelings of jealousy decrease but they're not going away, I am feeling bad. I haven't said anything to my girlfriend yet because I don't think it can do much good. Since they work together they will see each other anyway and also I cannot deprive my girlfriend of her closest work-friend just like that. If I say anything that might lead to their friendship to diminish, but probably for a short period of time only and she would probably start hiding details from me for fears of triggering my jealousy.

What should I do? I hoped my girlfriend would take a few steps back from this friendship on her own initiative but there's no sign of this happening.
<bot>: Girlfriend spends a lot of time at work with a guy who's a bit of a womanizer and this makes me jealous. They will be going away to  conferences in the future and this worries me in the long run.