It was not fixed... Niemczyk wrote about it in his book. More German players were food-poisoned, not only Grun. But Germany started the game with them and Niemczyk ordered girls to lose first set, they lost. And then German coach took poisoned players off to give them rest and Poland won. Simple as that. The match was not fixed, Niemczyk just let them win first set to see if they take Grun off and they did. It was rather good startegy than fixing...
He wrote something like "I would never fix the match. But you have to do everything withing the rules to help your team". Fixing is when 2 teams cooperate in it, not only one...
So it was stupid of German coach, he would have won the match 3-0 with food-poisoned players. Sounds totally legit.