It depends. If most of the employees know she escorts on the side which employees.. ie male or female. Girls chat worse than guys. If you decide to partake of her service she may tell ppl about your performance etc.
The thing to keep in mind is she your subordinate or equal or someone that works for the same company but has nothing to do with your department? This would be just like starting an office romance, if your her boss that could go bad for you, if your not her boss then if things go bad then you may come out with just a warning. Whatever you do, make sure there is no trail back to you by email or phone, and take the condom or make sure you flush it yourself. If things do go bad you want to make sure it boils down to a he said, she said think.
There has been a few past employees I would have like to ask out in my old job, to know she is an escort would just make it that much harder I feel your pain.