It's a grey area but will try to keep my opinion as black and white as possible
Yes - it's ok if both parties are unlikely to hurt anyone else in the process.
No - it's not to be encouraged if he is married for example and the potentially blossoming relationship may lead to something more with the consequence that an innocent third party gets hurt.
However, and here's my very own grey area, what's the difference between becoming close friends with an escort and becoming close friends with someone you met in a bar and just happens to be an escort ?
At the end of the day each individual person makes up their own mind on this. It's important to note however that if feelings DO develop between escort and client, I can't help but suspect that in the end it's the escort who will come off worse after he decides he's not going to see her any more because of the other family he's got at home.
The lesson here is that it's a word of caution to the escort just as much as to the client, if not even more so to the escort.