I don't think it's necessary for both of them to know how to dive, to run a dive shop. I think dive shops in general would be better off if they were run by business people who HIRED divers to work for them!
I don't know what field your daughter is studying, but there may or may not be jobs waiting at the end of the pipeline. And the other thing I would say is that one of my only major regrets, late in my life, is that I didn't chuck it all and do some adventuring when I was young and unencumbered and could have done it. There's no permanent damage from trying and failing . . . but failing to try is something you may not be able to remedy later. Your daugher may well discover that this is not a viable project, but as long as she uses some common sense about not getting into too much debt (or having kids) she should be able to abandon it later without taking too much of a hit. And who knows? Maybe she and the BF will turn out to run a first class dive business, and do just fine.