So i was at a buddy's place and a friend (not a photographer) asked to see my camera/lens. Without too much thought, i gave it to him to shoot while i went to use the restroom and grab some food. when i return, i noticed that he had a bottle of lens cleaner (for glasses) and lens cloth (looks like a generic cloth that comes with cases/glasses). he was cleaning my lens! he said he saw finger prints on the lens (i think he accidentally touched the rear element while changing lens) and wanted to clean it up for me. being a meticulous with my stuff, i politely asked him to stop so i can have it clean with the correct materials.
So my question, how soft is the rear element's coating? obviously, the best materials were not used to clean the lens, but after i cleaned it up at home, everything looked. i shined light through it and i got the different color reflections, like on my other lens. pictures look good still. can any harm have been done with his cleaning? how can i tell if the coating was rubbed off?