Don't assume that a science degree in biology, (physics, chemistry or whatever) means that you friend has had access the correct info or had the time to read over how to set up an aquarium or keep it going. Really brilliant people have not done so well with even guppies because they kept them in too small a tank or didn't keep up with frequent partial water changes using properly prepared water among other things. There are of course lots of other factors for slower growth such as temperature, specific diet, crowding...)

Fancy guppies are probably more vulnerable to trouble than are many other aquarium fish because of their finage. Fins like theirs are not "natural" and happen to be even more vulnerable to pollution and bacterial infection than many other aquarium fish.