I've never heard a pediatrician's opinion on this before. I know a lot of parents of kids my daughter's age are still helping with the wiping. We get these flushable wet wipes and they really advance any kid's ability in this area.
I'm surprised a pediatrician would say this unless the kid has had problems with UTI's. Usually, that would be a girl's problem, not a boy's. If your friend really disagrees with the doctor, she should go with her instincts because she knows her child better than a doctor he sees a few times a year. It sounds like she wants to keep this up, and I'm sure it's okay, too. But you might mention the wet wipes.
Holding it in at school doesn't sound like a good idea. You can't absolutely count on it, so why not foster that bit of independence?