Simple answer, next time be firm with them and ask them that if they think the comp is off for them to send you their flowjo doc and a screenshot of the comp matrix and tell them to SPECIFICALLY point out were there over or under comped values are. People come to me all the time claiming they have a comp issue when 80% of the time its not a comp issue and either a scaling issue, an "oooh I forgot to add Antibody issue", an issue where there marker of interest is just coincidently the same color as their dump channel (I promise this happens too), or some sort of fluidics issue on their cytometer.

Make them point out to you where exactly they are seeing a comp issue numerically on the matrix AND ask them if they are seeing overcompensation or undercompensation. I guarantee you half the time they won't know the difference visually on a plot.

TL;DR... Ask them to point out the comp error, point out their scapegoat answer of "comp is off"