The W203s (and most Mercs of that era) are part of Daimler-Chrylser's encheapening project, so one must be very weary of service history and the like. If he and his wife have their hearts set on this particular car, then they need to review it's maintenance and service history in full. Check for frequently replaced items (I saw a report on a '03 W211 with three A/C blower motor replacements over the span of three years) and also if there's not a good dealer network or a reputable independent shop around, I'd pass. Also, a loaded Mercedes means that it's going to eventually loaded with problems. The nav unit is fairly crap, replacement CD/DVDs are very expensive, power sunshades can and will break in a very expensive manner ($1000 for a whole assembly!) and make sure the A/C blows hard (our '07 W203 that my mom had in Abu Dhabi had very weak A/C).
However, if he can find one with very little equipment, then he will be much better off.
A car like that probably costs around $12-15k, there are better options out there.
*Cue X308 suggestions*