2002 Focus - Overcharging

my nepfew had the alternator replace on his Focus and a week or two later, his battery light come on. The light comes on about 10-20 seconds after the car is started. I checked the voltage (at idle) and it was 15.16V so I told him to take it back to the shop that replaced the alternator. He give them a call and the mechanic told him....
"15.1v is NOT that high - just ignore the light"
and then he said...
"it's a computer problem - focus is know for that"

how can he even say something like that????????

any thoughts? comments???

The alternator light coming on and the fact that it is 15.2 volts indicates there is something wrong there. About 14.3 should be the max for voltage at idle. I'd pursue it with the shop that did the work. It's a computer problem.......What?
I think one of the connections is loose. Don't call them on the phone, anymore. Just take it back. A motor mount has to be taken off and the engine jacked up to get the alternator out, but it most likely is just a loose connection.