Why not asking for a screenshot
Any problem which cannot be reproduced must be dealt with a screenshot and, if possible, a remote support session to see what is going on. So the actual error might have been human(e), handling it this way was unintelligent.
In this specific case you would have a screenshot proving your point instead of a wild guess what might be. Or, when doing your remote support, taking your screenshot for documentation where the problem lies.
In another case a GFX card driver or some similar issue can be the reason as well.