It can gub the seals in the high pressure (injection) pump, you cant compress diesel or petrol, only the atomised mixture, plenty of military engines can (sort of) run on either fuel, and Mercedes have perfected compression igniting petrol, but there are still some flexibility issues
Its too suspicious, even though the pump is ££££, someone would just have repaired it.
Lets face it - wouldnt most people get the car drained, refilled with diesel and then deny all knowledge and go for a warranty repair.....
If you do dunk a litre or two of unleaded in a diesel, just brim it with diesel, and keep diluting it by adding more every time you pass a gas station. Believe me, the regular fuel out the pump isnt 100% free of petroleum spirit!