over recent weeks car battery been weak, required several jump starts.
yesterday, battery light come on, then ABS and power steering then other lights, then car cut out driving at 5 mph. put brand new battery on and all lights apart from battery light went out.
my friend had a look today briefly, he tapped the alternator or something to see if the battery light would go out, then he said belt has snapped but alternator is fine.
I have a new belt but don't want to fit it unless I know the alternator is good first.

A shop tested alternator putting leads on battery and said alternator shows low reading at 12 V.
Could a bad serpentine belt (even if it had not yet snapped) cause an alternator to read 12 V and also there was no charge going out at all while revving the car but could that be due to a bad belt ?.

thanks