Sounds suspicious - the belt should last twice as long - yes it can snap and cause problems, but the belts should last over 100,000 which is why they recommend 80K, Im no certified mechanic, but its the first time ive heard of this. I would ask what makes them think it needs replacement, because I agree with your father in law, age and milage are fine.

One more reason to find a good independant German car specialist to take care of your ride - avoid the stealerships.