It won't be a single escort and a single client. It will be a single escort and every client from her appointment book that the police can locate.In most cases, there will be no proof of the crime, except the woman's testimony. So she can easily say he only paid for company, whether that's true or not. Or just say she's seen so many clients recently she doesnt remember that guy too well.
But this is still all hypothetical. It's very unlikely it would get to court like that simply for a single escort and one client.
You are saying she could easily lie, and it would be tricky to catch her, but why would she? She isn't facing any charges, the ONLY way she gets into legal hot water is by lying under oath. OK, so the police MIGHT not be able to prove perjury, but would you take that bet?
If you were that escort would you REALLY lie under oath? You have nothing to gain and everything to lose by lying. You can admit the truth without charge, but if it is proved you lied under oath then you go to prison.
I cannot imagine any lawyer ever advising an escort to perjure herself, all risk for no reward for her, so 9 out of 10 times she will sing.