Yes and no.
Sorry guys.
Because Firemouths have been indiscriminately bred for decades by large Asian fish farms much color can be lost in a fish. That is why I began line breeding FM's a decade ago.
Yes, as they mature more color will develop, but you won't always get a deep red throat/belly. you may get more of an orange washed out color.
Also, The red coloration may not extend to the base f the tail, it may only extend to mid belly section.
Furthermore, cichlids use color as a form of communication. So if the FM's are breeding you will see the most intense color each fish can muster. If they are stressed or threatened by a predator they can almost become grey fish and will assume a head down stance and hide.
Diet affects color, yes. Feed foods labeled "color enhancing" they contain carotenoids and will increase red coloration. Cyclops-eeze will help also. Believe it or not Paprika can enhance red coloration if rubbed on pellets. Finally add spirulina based foods too. The greens contain anti-oxidants that aid in coloration.
Water quality will help with size and unfrayed fins.
The environment should be as dark as possible. For best color use a black background, and a dark or black substrate. This will help to enrich the colors as they attempt to blend into the dark background.
Not all fish are equal by any means when it comes to colors. Let alone male/female dimorphism. Your friend can use the above suggestions and wait and see if he has a gem or not.