The place to find a room in a shared flat is
https://www.wgzimmer.ch/en/wgzimmer/search/mate.html?
Even for such rooms, one has to apply and sound convincing. It is worth reading every word in their ad, and addressing all of it. He should writing something about himself, his interests and focus, what he likes to do besides study, and should sound like a reasonable sort of person, to make the other students even want to meet him, all. It's a little like a informal job application. If he has previous experience sharing accommodation, that's a big plus, and he should say so.