As is often the case, a little more information may shed some light.
How many miles on the water pump? The water pump should last, on average, about 80k to 120k mi. Could be more, shouldn't be less.
What's the history of the cooling system? Unless it's been without incident there are several things to consider:
Have you ever changed the coolant? What brand and type did you use?
Have you ever had to add coolant because of a leak? How low did it get and for how long was it leaking. Has anyone ever added straight water to the system in an emergency?
Cooling systems are usually pretty bullet-proof for the first 4 or 5 years, and after that things can start to deteriorate. Check this
web article for a general overview on water pump replacement.
Make sure you ask for the old parts to be returned to you. Check for any radial play in the pump shaft. There should be very little, if any discernable play. Some will say that any play is grounds for replacing the pump, but on my own 300E I replaced the pump at 120k mi. just because I was in there for other repairs and it seemed like a smart thing to do. Upon removing the pump there
was noticeable play in the shaft, but no sign of any leakage past the seals. There may be some axial play, but that's not as critical. Check for evidence of leakage or staining at the water pump weep hole (see pic below). This is your first indication of a failing pump shaft seal. Here's an
earlier thread that notes some of the symptoms of a coolant leak on an M103 engine. Have you noticed anything similar on your car in the recent past?
And of course, there
is the chance that the technician somehow screwed it up, but it would be pretty hard to do unless he bent the shaft or cracked the housing, both of which would be pretty obvious. I can't really think of a scenario where he could cause some hidden damage that would cause the pump to freeze up and fail. They're pretty stout pieces.
Discuss your concerns with the service manager and ask him to give you a detailed explanation of what they feel the failure mode is, and have them show you on the pump itself. If they do this to your satisfaction, take their word on it. If not, let us know what he says and show us pictures of the old pump.
Hope this helps,
Gary