Being retired, I work this and four or five other Access forums for 6-8 hours a day, and have done so for seven years, now, and have never seen any reports of such! Also, not exactly sure what the phrase "usually rejected by access" means! To be honest, the teaching of Access in many schools gets shoved off on someone who wouldn't really qualify as a 'power user,' in Access, much less someone who should be teaching it!
I have no idea whatsoever what your instructor was referring to. As has been stated it is not a Reserved Word, in Access, and there is nothing wrong with the naming convention used, i.e using an Underline character to connect Client and ID. Using a Space between the two parts would be inappropriate, and can lead to problems, but not Client_ID.