Lying, cheating, stealing all comes with the territory of being an Addict. I can tell you from my own personal experience an addict is capable of change, but they need to get sober first.
I was all those things, then I got sober 25+ years ago and still continue going to meetings to keep by sobriety and because I do the right things today I dont have to lie, cheat or steal anymore. My life is pretty much an open book now - there is nothing to hide. I CHANGED.
The key thing about an addict getting sober and staying sober (one day at a time)is that they truly have to want sobriety and are willing to work on their sobriety (with a program - not just staying away from drugs)every day to learn how to live on a daily basis and deal with life without drugs. If they do these things they are capable of changing.
My 19 yo old, who is a drug addict, just got out of jail after almost 1 year (with no drugs). He was unwilling to go to any meetings or any type of program and within a couple of weeks of getting out he's right back where he was.
He's got a long road ahead of him to stay sober continue to change. I guess it depends on how much you feel for him and what you expect from him. If you are expecting all the things that need to change to be fixed tomorrow than you should walk away. If you have patience see how it goes.