This is perhaps a good way to alert him to the seriousness of the situation and force him to confront it and actually realize what's at stake here regarding his own life, and question whether it's worth that to keep this woman who's obviously not making him happy. But PT's right, I've got to dodge actually making that connection myself - the one between her being there and him not being happy. I've got to just sorta lead him by the nose a bit 'til he makes it himself.
The two can combine well, if you decide to take a more direct hand. You aren't limiting your options so long as you are excessively forceful with the "it is your choice, but common, talk to me" speech.
By coming to him and injecting the concept that what he is doing puts them both at risk, and that the risk is now tangible (a lot of people seem prone to doing something about it, including strangers), you may be able to get him to do something. Just be careful that something isn't "I'm keeping her no matter what". So the "I'm not happy with her" must come first, otherwise it could backfire in the worst kind of way.
In general, however, tell him that if he needs something (try to time it to imply that you will do whatever he himself is unable to do, such as turn her in), you'll do it, no question - but that you won't do it otherwise. By introducing both things (you won't do it without his permission, but you'll do anything he needs), you maximise your options, introduce the triggers and outs for him and give yourself credibility.
(I'm still strategizing in my mind, but that's likely the dominant one, given all the factors you have mentioned, and your priority over them.)