Thanks for advice, folks, I'm checking out the Schlumpf now....

To answer question, it's just some oil on the skin, which annoys her. I think she just bumps into the ring when jumping on the bike, we do a lot of riding in the city so there's always on/off the bike. I did the clean/wax once; a week later, she took it to LBS for something else and the tech at the LBS generously re-lubed it with black oil.
I thought the easiest solution would to find a ring guard; this has been my surprise, I haven't just found one made for the 50t SRAM, I kinda expected to find them easily.
If such doesn't exist, I'll clean it again and use the wax and then the weather will get colder and she'll start wearing biking tights which will hide the problem until next summer.