Going by what you say alone, that really is odd. She is doing nothing wrong and I don't understand why the police want to interview her. I'm wondering if a disgruntled customer or jealous ex-friend has dobbed her in. With the common misconception that prostitution is illegal, they probably think she's breaking the law and thinks she should get her comeuppance.
With the way she's working, and with nobody else sharing the flat, she's above board and isn't doing anything wrong.
If anything, she should be reporting that arsehole for threatening to rape her. That I would be most concerned about. Regardless of her job, it's something the police take very seriously. I would advise her to keep any and all texts, store them in a folder if she has to, and report them to the police. If they don't take her seriously, then she needs to take it further, to the vice dept. No way should she be putting up with that.