Growing up I heard the word employer and employee. I took French and realized employee was the past participle of the Fr. verb employer, to use, so it meant used or employed. Made sense.
Then I began hearing mentee as the person served by a mentor. Gee, mentor doesn’t even have the -er ending. Who came up with that? But now it’s universal. It comes from a French word mentor which is the name given to a character in a novel, kind of like Man Friday, and does mean guide or counselor but has no origin in a verb that could have a past participle.
Of course, that does not matter because now the pairing of -er/-or with -ee is entrenched to the degree that today a hospital volunteer introduced the young girl who was shadowing her as a “voluntee”, -er to -ee. Language is wonderful.
6/8/16 addendum. Nevermind. (Church lady). I went back to the hospital the next day and read the back of the shirt the volunteers were wearing. It said Volunteen. I had not heard the -n. Oh well.