No, in fact that is the major description of systems not optimized for 134a... they do ok at speed but are iffy at slow traffic speeds... the amount of air going across the condenser really makes a big difference if the system is not up to par... so a system starting to get low on freon but not to the point of shutting the system down with the low pressure safety switch might exhibit these symptoms...But I am not the one that brought up the possibility they stole the freon.....
Another possibility,,,, particularly on these sometimes strange MB's is a vacuum line misplaced... some of these cars use the heater to modify the passenger/driver temps... and so the heater flap could be engaged where hot air is blowing... as compared to ambient temp air blowing... don't know if he was making that distinction in his description or not...

ok,, let me restate that first concept.... on 134 systems not designed to make up for the inherent lower efficiency of the 134 molecule and low efficiency 12 systems (due to low coolant charge)this complaint of low speed warm air due to the lower amount of air flow across the condenser at slow vehicle speeds is common.