The course your friend took was most likely Fundamentals ... which is an excellent course. It is not, however, all the course you will ever need. Fundamentals focuses on certain core skills ... trim and buoyancy control, buddy skills, air management skills, propulsion techniques ... and it builds those skills around a very specific "platform" of diving equipment. It's an excellent class for recreational diving, or for entry into technical diving.
It is not ... however ... all the course you will need. It does not, for example, teach you fundamental rescue skills. It doesn't even touch on underwater navigation or search and recovery. It teaches you nothing about planning and preparing for limited visibility diving ... nor a host of other skills that are needful for a recreational diver who aspires to dive in a variety of conditions.
There is no "magic bullet" ... Fundamental is what it is ... a good start. A lot of people come out of it with an excess of enthusiasm because it opens their eyes to things they never were introduced to in earlier classes ... but, frankly, it was created as a remedial skills class for people who aspired to dive in caves or other types of overhead ... in other words, it gives you the basics of what will be needful if you should ever decide to go into the more challenging environments commonly referred to as "tech diving".
I've taken the class ... twice, in fact ... and gotten a lot out of it both times. I've taken more than a dozen follow-on classes since ... from NAUI, TDI, IANTD, and NSS-CDS as well as several workshops that were "non-denominational" ... and I'll continue taking classes as I identify new techniques, equipment, or environments that I want to learn more about. I've taught NAUI classes in a shop that offered GUE ... and have heard the "why take anything but GUE classes" logic from others who were enthusiastic about what that particular class taught them. The best answer I can offer is "because GUE doesn't offer all the classes I want to take".
A popular diving magazine has these words at the top of the front cover ... "A Good Diver Is Always Learning". No one agency teaches everything there is to learn. No one approach to diving offers everything you'll ever need ... not in the broader sense of "you". We're all individuals, with individual goals, insights, learning styles, and interests. GUE may indeed be all your friend will ever need ... you may discover, as you learn more about diving, that there are other classes taught by other agencies out there that will open vistas for you that GUE does not. Or you may ... as I did ... come to realize that adherence to that one style of diving is too restrictive to scratch the itch you have for diving in a way that you want to pursue your interest.
Even the GUE instructors I've met will tell you to approach it with an open mind ... they are not just referring to what they will be able to teach you ... they're referring to diving as a whole. They don't want blind adherence to a system ... the most important thing they're trying to teach you is how to use your brain ... it's a valuable skill, and not one that always applies strictly to the context of a GUE class ...
... Bob (Grateful Diver)