I would try goof-off if its really stubborn, if it is stained wood I would look at some wood cleaners/ and possibly sand and refinish portion to match depending on area and quality of unit.Also give TSP a try.

She is very stubborn and really whats something she can call her own.

Okay, maybe, but not that great.So since you asked for help, I'm going to offer my stubborn, ornery little rule, borne of hard personal experience. 

Older people are more stubborn & if they've been living in the property that long, they're probably going to die owning that house.

It is likely the seller is stubbornly sticking to an unrealistic price, but months on the market and possibly weeks between showings might increase their motivation.

Engineers tend to be a bit stubborn.

I have a full time job right now but I'm focused and pretty stubborn when I set my mind to something.

I'm kind of a stubborn 65% - Repairs kind of guy. 

If he doesn't bite one of two things is the case, either he is just stubborn and will not buy anything not in his very narrow mindset so there is no point unless it is one of those types of deals or, and I mean this with all due respect, your investment case had some flaws in it. 

I will truly say being stubborn as an ox with the belief if there is a will there is a way was  the pinnacle behind these deals!