An eminent US scholar in mentoring recently said at a conference “There is no such word as mentee!”, arguing instead for the word protégé.
By the same logic, of course, there is no such word as protégé (French, from which it might be presumed to have originated, has a verb, protégér, from which come protecteur and protectrice, but no noun for the person being protected.)
In reality, of course, words become real when they are accepted and used, either generally or within a specific context – regardless of their origin or “legitimacy” (the assumptions people make and try to impose about the rules of language, often based on misunderstanding of how language evolves). Another acid test is “Is it in the dictionary?” – which both mentee and protégé are.
Equally, a concept typically does not exist until we have words to describe it. Some isolated tribes have no concept of mathematics, because their language has only one, two and many. The evolution of word and meaning is a circular, systemic process, not a linear one!