If you want to second-guess him, you need a voltmeter & do the checks I said. (Not that I can guarantee he'll listen to you...) You can also see if the battery voltage goes way down overnight when you DON'T charge it. If the battery is dying on you, that's not likely the starter's fault.

If you have the car back home, try this next time. When it doesn't start, rap on the starter with a broom handle or something. If that makes it work, then it's the starter. If that doesn't make it work it really doesn't prove it either way. (Did your dad get a new starter or a junkyard starter?)