It could be a hemorrhoid; you might look those up and show her a web page describing them. Often they are smaller, like a bunch of grapes, but sometimes they can get big enough to need medical intervention. (A friend of mine once had a "hemorrhoidectomy" for one that got out of control.) The idea of needing a procedure to deal with it won't soothe your friend's worries, but she's got to be worried already about what it is, after a year. So, she should be seen by a doctor. She is unlikely to have anal cancer because she is so young and anal cancer is rare, but that is another reason she should screw up her courage and see the doctor. She needs to rule out things more serious than hemorrhoids, as well as get this issue addressed.
To help with her distress, the person making the appointment (whether you or her) might explain to the nurse that this is very hard for her because of her PTSD, and ask if they can call in a prescription for one (single) Vallium tablet, for her to take before the appointment. It will help.