Year & mileage don't seem to add up, so careful research of paperwork such as servicing records, any emissions tests etc. plus polite questioning of seller and looking for signs of higher mileage (general condition, engine noises, exhaust smoke, worn carpets/seats/pedal rubbers, wear & tear on steering wheel & gear knob etc.).
Full service history with regular oil changes/services is a must. Despite low mileage, due to age if there is no hard evidence that the cambelt & cambelt tensioner have been renewed (sticker on engine, invoice from garage) then this may need to be done soon. Adjust price down accordingly if this is the case.
S40 suffers with rubber engine mount bush wear, fragile trunk lock mechanism & a few other traits - it's well worth taking the time to read through the S40 1st Generation section of the forum.
There is also a helpful guide on things to look out for when buying a pre-owned car, under the "general Volvo non-model specific" section of the forum.
Good luck - if it's genuine mileage & fully serviced you may have found a real bargain.
He's probably already bought it by now.
I've had a 99 S40 2.0T for about 3 years now. It's done 85000 miles so far, never had any trouble with it.
The only thing I can suggest to look for or rather listen for is a creaking noise accompanied by a quiet knock when you freewheel on a bad road surface. This is a mix of worn engine mounts and top mounting brackets (front suspension parts).
Listen out for the same noise when pulling away too.
I have been told by a volvo garage that this was a particular weakness in the 99 model.
Us towing horse boxes the length of the country probably didn't help much.
Apart from that it's so far been a fantastic car, very comfy practical car, quick too, and yes we have been unfortunate enough to test the safety features for real. One 30MPH rear shunt by a VW Sharan. The VW was written off. And we've been T-boned by a little fiat.
Both occasions my whole family were in the car, not so much a bumped head on either occasion. And drove home afterwards.
But be very wary, let him know that if he does buy one, he'll probably never want to drive any other make of car. Many a Volvo owner will tell you this.