I cannot believe I am reading this!!!
LOL I thought I was the only one this happened to! I fell and broke my tailbone several years ago. I had severe bruising on one side of my buttocks and hip. The bruising and pain were severe for several weeks. Months later, my roomates noticed the "dent" on one cheek. I did what you did, massaged it for weeks. It didn't get better, so I went to the doctor. (Some may say I am vain, but I didn't like how you can see it when I wear something sheer).
I got bad news however.
Sorry. The doc told me that I fell so hard, the muscular tissue underneath the bruising "died". He said that this can happen when the trauma to the soft tissue is severe. He said that the dent is there because the dead tissue was absorbed over time by my body, and that left an empty area, hence the dent.
I am sorry to say that all these years later, and MUCH massaging, ect. it is still there. My doc said that all he could think would fix it would be plastic surgery. But, I am not going to go there.
She should still see a doctor to confirm the diagnosis. I went several months later, and often think if I had gone sooner, it could have been treated....but I don't know. Did she ever see the doctor? I was surprised to hear that my tailbone was fractured, even though it did hurt very bad. She most likely broke hers as well. Keep in touch and let me know what you find out. At least tell her she is not alone!