I am by no means an expert but I do know that the alternator size requirement is a function of the amperage draw of the various electrical accessories on the car. That's the place to start. What's the max potential amp draw assuming likely accessories to be on at the same time?
Before looking for a higher amp alternator, I think I'd make sure the current one is doing what it's supposed to do. If your friend hasn't a clue about this stuff, then he'd probably be wise to seek some professional advice/troubleshooting.