As a non-resident traveling to CH each week and truely remaining living in Belgium I agree, but none of that was indicated in the original post. From the post, he is living in switzerland for 3 months and he is working in switzerland for those three months (and not as a regular employee of a foreign firm in CH for a while as part of his normal capacity).

From the OP, it seems that a 'real' answer is desired, not the 'wink-wink' answer that will cause the least problems...what is done with that information is up to the OP's friend - I would say letting dogs lie is the the easiest, he can always claim ignorance if there is anything untoward and certainly the firm would have to take some responsibility, though it is very unlikely a problem would come up.

Of course for the real answer you can contact the swiss tax authorities, they get all creamed up over this type of stuff I'm sure...And if Richard is right then there is nothing to worry about.

Andrew