The settings that might be causing this could be in the browser for security or privacy, or it could be some third party software that is doing this. Do you know the browser and OS that this user has? You might be able to replicate the issue they are experiencing with the exact same setup. Then you would be able to check on some things and do some testing to determine if that is the issue. Being able to reproduce this issue to some extent will be the first step in determining what the cause is. Do you have access to the clients machine that you could spend some time on trying to test out settings? This might be a good idea if you do not have a way of reproducing the result yourself.