But ... what if he crashes? I don’t think he has any insurance.
A friend of my son is visiting and has asked if he can use my bicycle.
It really depends on which country he lives in.
There might also be the issue of what or who he crashes into.
What if he's walking on the street and gets hit by someone on a bike?
Anyone visiting another country needs to make sure they've the correct health insurance. There may be a mutual healthcare agreement between the home country and Switzerland.
He needs travel insurance. I don't think you'd be liable as the owner of bike
Are you worried about health or liability?