Hummm : if it wasnt leaking before your friend changed the thermostat , it only makes sense that it's leaking from around the thermostat houseing or a radiator hose he might have taken off .
Don't let the shop sell you a water pump job !
i would have my friend that done the work check himself . the thermostat gasket needs some gasket sealer around it or the thermostat is not setted in the grove right and the housing is not going down straight , if thats the case and he just tries to tighten it down some more it will break the housing , easy mistake to make once , i know hehehe
call you friend and tell him whats going on

