He feels like a stranger because in some pretty big ways he is. He has this second life he's living. He is not only high half the time (or more), so not in his "regular" mind, he is out buying drugs, talking to drug dealers. Spending money you could use to buy the house you spoke of. Sure he has credit card and other "legit" debt. I would hazard a guess that a lot of that debt is cash advances.
When he says he is doing something as innocent as say, picking up pizza for dinner, will you now wonder if that's pizza with a pit-stop for some cocaine? For six years he has been lying to you.
Sadly it's not that uncommon of a story. I've seen quite a few people post here that their SO is a drug addict or even an alcoholic (which is easier to spot, generally) and they didn't know.
There is a lot to consider, as Suki said, no need to make a decision in the next few minutes, take some time to consider all of this, you are probably still in shock.
Recovery takes times, while he might get sober right away actually recovering from drugs takes a long time. Many addicts use it as a coping mechanism. Don't be surprised if he starts going through withdrawal or you see personality changes. Recovery is hard stuff.
If you were considering having children, that might also be a consideration here because you would probably want him to have at least a couple of years of solid sobriety before adding children to the mix.
I agree with your friend, look out for yourself here, protect yourself and your feelings.