$4000-$5000 isn't an unreasonable asking price in my opinion. They're very rare these days. Especially super clean examples like that one appears to be.. If it's been a Texas truck for nearly all of it's life, then the value will be higher just because it's a rust free specimen.
If she wanted top dollar, and was willing to work with vehicle delivery companies, putting that truck on Ebay might get her far more money than she ever imagined. Or it might flop. You never know..
It really does depend on a lot of factors. Usually the diesels I see, everything under the hood is coated with a thick layer of diesel and sludge/dirt.. The interior being in excellent shape will increase value a lot too.
I see 1st and 2nd gen Bronco II's selling for $15,000 today, so asking $5000 for a rare Ranger that's super clean isn't out of line at all.
If I was currently working, I'd have no problem giving $4000 for that truck right now, but I have to watch my spending.