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Mother softens up during her time in "out with". He prejudices begin to soften as she spends more time with the maid Maria and Pavel. I think she really does feel thankful to Pavel for helping her son.
Pavel helping Bruno when he is injured is an important event because it makes mother realise that Jews are not all bad, and she questions her own prejudices against them. When she thanks him for helping her son, this is the first action she makes to show her acceptance of this realisation, and the first in her changed attitude towards Pavel. By seeing that the Nazi believing race is not really superior to the Jews, mother has lost some of her pride, and becomes more humble.