Check your temps. They might have dropped while your friend was 'looking after' him. This could cause him to stop eating.
If he is just being fussy, then you could try cutting the mouse in half (the pet shop was right), braining the mouse, leaving mouse with snake overnight...no disturbances etc. There are a million and one reasons as to why your snake has stopped eating but if he seems ok...not off colour so to speak
, then it should be OK and he might just start eating again as quickly as he stopped. Hope this helps, I know how frustrating it is to have a non-eater!