All the advice so far is sound.

Unless there is a punitive clause (ie he must pay 50000chf for simply going to a competitor) his employer must actually suffer a demonstrable loss to sue him. And it is going to be highly unlikely to be worth them trying unless there are large sums at stake.

He should also take care not to fall foul of any confidentiality clauses, about business knowledge etc.

He could also consider raising it with the prospective employer and getting some contractual protection in case he is attacked, but that would only be if they were really keen to have him specifically, so to be done with care.

D