Ratzinger,
For what it's worth: You described the person you want to share a place with as your "coworker," and not a friend. I think that's significant. Sharing living space with a friend can be difficult enough, let along someone you are not really close with on a personal-friendship level. How much do you really know about him and his background, character, etc., outside of work? I assume you know even less about his girlfriend, except she has a serious character flaw.
You would be smart to pass on this arrangement on the face of it.
K