Mentee

Yes, mentee is in the dictionary and it means a person guided by a mentor, but if you ask me, it's a badly formed word (and I will never use it). To me it should be mentoree (which I wouldn't use either), since the suffix -ee is normally appended to verb forms and the verb is mentor. The dictionary has a good explanation for this suffix: a suffix forming from transitive verbs nouns which denote a person who is the object or beneficiary of the act specified by the verb (addressee; employee; grantee); recent formations now also mark the performer of an act, with the base being an intransitive verb (escapee; returnee; standee) or, less frequently, a transitive verbattendee) or another part of speech (absentee; refugee). It doesn't say anything about the root of words, and ment- wouldn't be the root of mentor anyway, the whole word is its root.

Addition: yesterday I came across tutee in a book, which is also in the dictionary. I guess people are creating such words by taking off the suffix -or and adding -ee. I still don't like it, though.