My Friend Has An Indentation To Her Muscle And Skin

My friend has an indentation to her muscle and skin to her right mid upper arm, it started small and is about the size of a quarter now. She said it started about 6 months ago. Now on and off it started blistering and weeping. No one can tell her what it is the doctor said he's never seen anything like it before. It wasn't weeping when he saw it. What could this be? Is it serious?