When the Macintosh is set to record in Mono mode, it adds together the signals of both the left and the right inputs and averages them together into a mono signal. This averaged mono signal is a lower level signal. By summing and averaging the right and left inputs into a mono signal rather than picking one input no information is lost from either input.
If there are concerns that the input level of mono recordings are lower than those of stereo recordings there are a couple of workarounds.
Increase Source Signal Level
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Have them increase the level of the signal that they are supplying to the Macintosh. This compensates for the reduction due to the averaging to a mono signal.
Use Hardware Spliter
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If you are supplying a mono signal to the macintosh, you can use a splitter that would take the mono signal and split it into two signals. The splitter would be a Y-cable that would have a connector on one end for the signal input (3.5mm, 1/4-in or RCA jack) and the other end would have two RCA jacks to connect to the sound input jacks, or a stereo 3.5mm jack which could be connected to the Microphone input.