Sarah, there are lots of different lighting systems, they break down into three general categories, continuous, small strobes and studio strobes. Within each category there are dozens of makes/models. I think most photographers will have a basic idea about capabilities but not many specifics unless they are using the equipment regularly. I have several models of Canon flash, and I can program them all to get what I want. Don’t ask me to do anything with a Nikon flash, the only Nikon camera I have is an old film SLR and I have a third party flash for it, I have never owned a Nikon flash. I understand some of the similarities and differences because I have sat through Nikon presentations, but would not attempt to use one without reading the manual. Neither Nikon nor Canon flashes are anything like my Bowens studio strobes. Power is different, size, weight, controls, the radio that fires them, sync speeds… all totally different. Yes, you can get a softbox for a Bowens strobe, and you can get one for a Canon or Nikon flash, they mount differently, have different construction, different baffles inside, and for small flash, Westcott seem to make an endless assortment of shapes, sizes and constructions. I have seen two or three different models that are just for hot shoe mounted flash. Are they worthwhile? Have not a clue, never bothered with them. I tried a Gary Fong Light Sphere, it is a pain, so I don’t use it.
If you are used to studio lighting with everything up high on stands and only inches from your model’s face, I can see questions arising about what to do with a speedlight.
If he is really a fashion photographer, I think he must know something about softboxes since most of them use them, but the softbox that goes on a 500 W strobe is not the same one that you put a flash with a guide number of 60 into. I wouldn’t take either to a wedding unless I was setting up a photo booth.