A lot of my counsel will be similar to what Lynx says, your relationship is with this woman not her adult son, so how this impacts her is what your focus should be on. I would add that I have one of those mothers who talks tough and intends to do right by her kids, but I also don't think that she's going to let her kids not have a roof over their heads as long as she's able to provide one (we're both out of the house now probably for good but life happens). And so I would advise you to let her tell the story of how it's been with his addiction and in listening you will probably have a good idea of if she's got a good gameplan for dealing with all the crap that comes with having an addicted family member and if she's likely to stick to it. And I'd also say if you decide to stay, then be ready to be someone speaking truth into the situation when she gets confused or guilt tripped by his attempts to manipulate her.
Other than that, what you said makes me think that "peace with this person" is some sort of code for someone who has no major issues in their life. While you are right to recognize the gravity of the situation, if you're looking for a person who's got it all together and has nothing that could be a serious problem down the road, you'll probably be single a long time.
Oh and since no one has said it, I'd pray for a lot of wisdom and that God would give me a very accurate judgement of my own strength so that if and when I decide to stay, I'll be able to see it through.