One thing I can guarantee is that every prostitute has had bareback sex at some point in their lives, even if they don't offer it to punters. Same for punters, even if they always rubber-up with prossies.
The answer is to always be careful, regardless of feedback. That same bloke could easily have shagged her and not left feedback, and you'd be none the wiser.
I'm not sure how much greater the risk is having protected sex with a prossie who has had sex with someone else who had 2 steps removed bareback sex (maybe).