This is not good. Something is wrong. I wonder if there are any REAL German citizens who will stand up and exhonerate the dad. HE DID THE RIGHT THING. If he had killed him it would have been good.
If the groper were not intoxicated and posed a real threat of physical harm to himself or his daughter, then violence would have been an option. But if he had confronted, detained and notified the management and had them call the Polizei, the perp would likely have been charged and dad would be free to go.
That said, I do not believe I would have done that and probably would have given the drunk bastid a really good tune up. But...I'm a large guy with some skills and a boxing background and a very short fuse. But the right thing to do in such a situation is to not resort to physical violence. Unless circumstances demand it.