I agree he should not have been parked so close if he was intending to run his generator extensively, on the other hand it is Walmart where there are no rules since it is free overnighting.
On a couple of occasions we overnighted at Walmart or casino parking lots and although there were acres of open parking and RVs scattered throughout, on one occasion someone parked right next to us.
Suffice to say, we ran our onboard genny although they didn't come over to complain, which would have been a waste of their time anyway. If I don't want to hear generators I would never go to a place that allows their usage.
At Walmart there are always several commercial diesel trucks running throughout the night keeping their contents cooled or frozen anyway, quiet and overnighting at Walmart is not something I would ever expect.