I'm not a Mac person, but I imagine that most audio editors would allow you to convert the stereo file into 'dual mono' so that each channel can be edited separately.
Then I would cut out, say, 5 seconds of the offending channel and highlight and copy the corresponding section of the good channel but with a good 3 or 4 extra seconds (or more) at each end.
Then paste the good section over the gap in the good channel. Most programs will automatically perform the crossfades at the areas of overlap, so you'll just need to make sure the clips are synced up - zoom in and match up peaks on the waveforms.
There may well be a more straightforward way of doing this but I think this is what I would do as a first attempt.