Your friend will probably need to apply for official 'independent' ie self-employed status with AVS / AHV. This is not always straightforward as one must have contracts with numerous different clients. Contact them directly for info. 

AVS / AHV ?  so in case he has only one contract what can he do? it seems very weird this requirement...

You start from OCAS close to Cornavain - they are nice and will explain everything:)

Reffael - yes, it is weird. They make you jump through a few hoops and you need to meet their criteria. But as Kris says, they are available to discuss with you and explain.

and what do you suppose to do until u have couple of customers - how r u suppose to work with the first one? no one will work with u if r unable to give receipts for the payment.

U actually need first clients to register as independent at OCAS & u can alwasy issue an invoice

Actually most important is that you have legal residency? After that, then you can work or work as an independant. And Independant requirements must still be met, or you loose residency status/permit.

There is a good summary of the steps to being self employed in Switzerland on ch.ch, Link below.

AS Dave G said, most important step is to obtain a pemit with right to work as an independent.

https://www.ch.ch/en/becoming-self-employed/