NO-NO-NO-NO. Don't let that PB near the thing. He, she, them are IDIOTS.

Cleaning that ring is easy and safe. I use a scrub brush I got at a poolstore that looks like a coarse foam pad. Make a thick paste of baking soda and water in a bucket and go at the ring. Others on this forum have other methods of cleaning it off and I suppose there are safe cleaners available but I never used anything but baking soda. In years on this forum I have NEVER seen acid used on a vinyl liner for anything except lowering pH. Then, you keep it away from the liner.
Al