Send him to
Edmunds to find out how much such cars should cost.
Maybe he could find a 2-4 year old A4 or 3er with that mileage for that price, but barely. He'd have to find a car with absolutely no options. Better chance on getting the Audi cheaper, as they came out with a new model last year (B6 platform), which obsoleted the old model (B5 platform). 4 year old (1999 sedans) BMW's are still current model (E46). Might have better luck with 1999 BMW coupe, as those are previous generation (E36)
If he's serious about the used car thing, buy a month-to-month online subscription to Consumer Reports (and cancel after one month) to get reliability data from there.
Check out the following forums:
Audi
BMW
The forums will give information on known issues with the cars.
Carfax is always recommended.
Good luck.