In my 2 GM vehicles the AC gets colder as the refridgerant leaks out over time. After about 2 years they both become very effective coolers but the compressors cycle more and more. Eventually they become ineffective and require a recharge. Contrary to intutition, when fully recharged they are not as cold as when slightly discharged. The caller indicated the AC was still cool but not cool enough. This is a clue to the possibility he had overcharged the system.
I would like to hear the technical explaination on the slightly discharged system putting out colder air. I do know a sign of a low charge is lower low side pressure which does mean lower temperature. My practical experience does confirm overcharge equals higher pressure which means higher temp.