What I personally think about a meditator who visits prostitutes is that they haven't learnt very much. I don't believe in encouraging that industry. I don't believe it is showing compassion for anyone involved in it.
What to say when talking about it? If the person is open and they admit to doing this and they are open to take some advice, talk to them about having compassion for themselves, for the pain and suffering of the people involved in the industry, and for what they themselves are supporting. Try to get them to understand the compassion element and get them motivated to turn away from doing such activities.