The client of a sex worker is colloquially referred to as a client. The etymology goes back to the Middle High German "vrîen" (to advertise, to free). In professional discourse, the term "customers" or "clients" is increasingly being used.
In Switzerland the use of sexual services from adult, voluntary sex workers is not punishable. The john is only liable to prosecution if he uses the services of minors (Art. 196 StGB) or of people in a recognizable predicament (human trafficking victims, Art. 182 StGB) - even if he only accepts the predicament.
Politically, there has been discussion for years as to whether Switzerland should adopt the Nordic Model (ban on purchasing sex), which criminalizes the purchase of sexual services across the board. Sex workers' associations (ProCoRe, Aspasie) reject this - it pushes the industry into illegality.