Corolla or Camry wagons are dirt cheap. I bought a 97 'rolla with 41k on the clock for $4000. That's not typical, but some little old lady just got too old to drive and put it on CL. 35 or better MPG.
Honda civics and accords are right up there as well, but they tend to hold their value longer = less car for the money compared to a 'yota
Diesel SUV? Not exactly pinnacles of low maintenance, but if you can convince him to change the oil at least every 5000 they'll run forever. The GM 6.2L diesel is a workhorse and it can pretty easily get 22+ on the highway. You can often get a suburban or blazer with a 6.2 for under a grand. For $7500 he should be able to find a Powerstroke Excursion with 150k on the clock and it will still be good for another 150k. Insurance would be a tad higher, but not bad.
Pretty much any of the japanese FWD econoboxes would do. If he wants more cargo area than the sedan/coupe offers, he can hold out for the ones that come in a wagon. I'd say the little SUVs would do, but a CRV is just a civic that costs more, gets worse mileage, handles poorly, and the insurance is a little higher. No need to pay extra for less car.
older 5-series wagon? That will be dancing on the edge of the 100k mileage cap, but again, those engines are well-known for their longevity.
Ooohh... the ultimate idea... V6 GM G-body. They don't get great mileage, but they are dirt cheap, insurance is next to nothing, and they're super cheap to fix. El Camino, Malibu wagon, Cutlass, Grand Prix, Monte Carlo. Some of them even came with the Olds 350 diesel which isn't as bad as everyone thinks. Fix the head bolt issues and you're done. Give it clean fuel and enjoy 30 mpg.