Bad Idea guy.

He´s not a friend of both, he's a co-worker of her.

Never mix sex with work. Never.

And if he's a co-worker, he's with her a lot of time... bad idea friend.

If you want a threesome, well, it's ok, but try to find a guy not so close only to your GF, or maybe you could lost her. Better if it's a guy/girl with some degree of separation.