The term Freier colloquially refers to the client of a sex worker. The etymology traces back to the Middle High German "vrîen" (to solicit, to court). In professional discourse, there is an increasing tendency to use "clients" or "customers".
In Switzerland, the use of sexual services from adult, voluntary sex workers is not punishable. The client only becomes punishable if he avails himself of services from minors (Art. 196 StGB) or from persons in a recognizable coercive situation (victims of human trafficking, Art. 182 StGB) — even if he only accepts the coercive situation.
Politically, there has been a discussion for years about whether Switzerland should adopt the Nordic Model (prohibition of purchasing sex), which criminalizes the purchase of sexual services outright. Sex worker associations (ProCoRe, Aspasie) oppose this — it would push the industry into illegality.