If he has been living in the house for 20+ years and is a moderate to heavy smoker, there may be rooms where scrubbing, sealing and painting simply won't keep the chemicals from coming to the surface and you will end-up pulling down the drywall and hanging new.We renovated a house some years ago where the former owner had a favourite chair in one corner of the living room where she sat and smoked.  

I know chemicals are expensive for large pools that are well used or with debris getting in, trim branches yourself.my nickle's worth

Last suggestion is to find a couple chemical suppliers in the area and talk with the chemical reps.

How to keep a rental house with a toilet flooded , a higher curb than the floor , company does trust . this happens every year with tenant 

Depending on the slab they either trench the home or bore holes and spray chemical into it.  

That makes it much easier for me-I'm chemically sensitive, and would have to buy no-voc paint ($$$) instead of regular, because I will be in it for another month or two.

We actually own and operate a high-end luxury home cleaning business.We hired professional cleaners upon move-out & also had the carpets professionally cleaned at 6 months living there and again upon move-out and used proper chemicals and a professional power washer to clean any oil stains from the driveway we may have caused with our old work vehicle.Our landlord charged us the following odd, ridiculous charges:$3.33 To empty out the ice from the freezer ice maker $1.08 To screw a lightbulb in the bathroom vanity$30.00 To remove a nice black metal paper towel holder we had put in the kitchen cabinet$6.67 To fill and paint a tiny minuscule hole barely visible to the naked eye in the kitchen pantry door$3.00 For a smoke detector battery$3.33 To remove a small piece of duct tape from a wall switch in the kitchen we had placed there because every time someone flipped the switch accidentally it would turn the dishwasher off.

Oh, if you see flames it's too late, the weed grower will have started and electrical fire messing with the wiring to bypass the meter, and the meth lab will have had a chemical explosion of some sort.

I'm a 30 year old chemical engineer with a steady job and generous income. 

The concrete looks discolored from a chemical.