Be prepared for sheer chaos in your life. I hope the best, but I do not think this is a good idea. He will drive you crazy just like he is driving his parents crazy. Do not leave anything of value laying around, hide your car keys, and bank cards. Do not let him be there while you are not home. Realize that an addict will do ANYTHING to get drugs, I know because one of my best friends became an Oxy shooting addict.
Also, it is not like booze, it is hard for a non-addict to tell if someone is using opiates, so it is not like you can kick him out if he comes home high - you probably will not know. Just be careful, you are not equipped to deal with him, and the friendship your husband has with his friend could suffer because of this.
Here is a great post about what to expect from an addict, and it is very true:
http://www.soberrecovery.com/forums/...ddicts-do.html
I love my friend so much and I also feel for this guy you are taking in, but I am with Sugarbear on this one. Send him to the Salvation Army rehab.