When you complete the application form for a permit you sign to state that everything you have written is correct and if it is not the Swiss have the right to withdraw your permit with immediate effect. With respect to taxes this is covered in the bilateral tax agreement between Germany and Switzerland. Within this exactly this situation is covered. It is worth noting that both Germany and Switzerland have taxation on global earnings. It is also worth noting that tax payments become time barred after ten years. It is further worth noting that both countries will tax on the full amount retrospectively, and for tax avoidance, which this effectively is, the agreements between the countries do not apply ie you are taxed by both countries fully!

So what is your friend risking? Put simply German Tax will be around 40-45% of combined earnings. Swiss Tax will be around 25% of combined earnings. Total tax burden is then circa 65% of total joint earnings. Which is more or less all of it when you take out social contributions and pensions.

So why not ask him how he will feel if he gets a bill for his total salary for the whole length of time he has been in Switzerland! And then ask him if he wants to correct this oversight.

Upon correction the Swiss and Germans are very lenient ie they will simply recalculate the taxes they would want and check to see if this works out as a plus or a minus. It should not make much difference in reality.