My wife and I went through something similar in 2013 when her father passed away (life long drug user also). He left my wife a modular home, but her still living grandfather owns the land. The modular was in disrepair also and all of his crankster shit was left behind.
I took 2 weeks off work and we used that time to get everything out of the house. We saved stuff that meant something to her, donated what we could to a local thrift shop, and threw away a dumpster load full of crap.
We ended up letting her cousin stay in the house because he's handy. He did some renovations, and then ended up getting booted by her grandfather for drug activity. We ended up selling the modular back to her grandfather for like $1200 and now he's letting his new girlfriend, who is 30 years younger, live there. It's been nothing but stress for us, since even before he passed.
Coincidentally, today (Feb 1st) is the 4 year anniversary of her father's passing.
If you ever decide to have a BARF renovation party, I'd be willing to help also. I've got a truck and can help demo and haul stuff. I don't know squat about plumbing, electrical, or HVAC, but I can hang drywall and help with flooring since that stuff is pretty easy.