[JayG]

+1 on finding a mechanic that actually knows Porsche Cars and works on them regularly

Do you know what exact water pump he installed? Hopefully it was not some autozone/napa special with a metal impeller. Lots of threads on water pumps. In short, a metal impeller while is sounds like it would be better as it wouldn't break up into little pieces is a problem because if the bearing wears and you get any wobble in the shaft, it will act like a grinder to your engine block. Best bet is a Pierburg WP which makes them for Porsche and is an identical part

Rex is correct on the coolant. If different types of coolant are mixed, it can cause a lot of problems. $35 is really cheap for Porsche coolant, most likely around $40-50 a gal.
If the WP and thermostat were replaced, they should have also flushed and refilled the coolant