Its true, she wont remember who her biological mom is.
But you are her mom now and she will always remember you
Maybe they made a mistake and took on more then they could handle when they bred their dog, they should have done research on breeding and yorkies first but sadly alot of people dont do this.
I always believe that everyone deserves a second chance, if they say they wont breed again then maybe they have learnt their lesson.
If it were me I probably would stay in contact but I wouldnt become 'good friends' with them, the way I see it is that if you stay in contact you can educate them on yorkies.