Men who use prostitutes are colloquially called johns. Is there a specific word for women who use prostitutes?
I've seen jane is used for a female john but it is not as common.
John is used for men, however you can find gender neutral definitions of the term as well. For example, Newspeak (Routledge Revivals): A Dictionary of Jargon (By Jonathon Green) mentions that it is used for both male and female.