You situation sounds so familiar to me. I too, gave my "friends" (in quotations because I did not think friends could do things like this) children because I wanted them to be happy. First a son and then a daughter (who is full-blood sibling to my two raised sons and is my and my hubby of 11 years blood daughter) I too questioned my decision. I DID NOT try to get my children back. I was promised a very open relationship and that my children would know me always. They decided after "our" daughter's adoption was finalized that they no longer wished to be my friends (didn't want any "interference" or "confusion"). Now I am without my "friends" AND without two children who I still love and worry about everyday.
I AM NOT saying that your friend would do this to you, just letting you know that I can relate. My advice would be to do what your heart tells you. If your heart tells you to get your daughter, I think you should do that. Before it is too late. I didn't listen to my heart. (I even ignored red flags that were waved in my face because I BELIEVED that I was doing something kind and compassionate.) If it is not too late, get her back.
If your friend is truly your friend, even though she would be hurt and even though she may even be angry, if she is REALLY your friend she will eventually forgive you and support you. She will ALWAYS have a very special place in your daughter's life even if you raise her. She sounds like the perfect Godmother/"Un-Aunt".
I also wonder if you have asked yourself if your friendship will survive watching her raise your daughter? If you are having trouble watching them with her now, how will it feel when you disagree with a parenting choice they make? You need you ask yourself these questions and answer yourself HONESTLY.
PLEASE, unsderstand, I have the utmost sympathy for your friend's situation, however, your daughter is YOUR daughter and you as you said yourself are able to raise her. You do not owe your friend your child, nor are you responsible for her reproductive issues. I know you love your friend and wanted to help her but your daughter should be with you and her siblings if there is no real reason you shouldn't keep her.
I hope that your friend would understand and I hope that she would eventually able to be happy for you and have peace with your choices. I firmly believe that things happen for a reason (yes, even in my own situation - perhaps I am meant to be an example of what can go wrong. Who knows?) perhaps your daughter was not truly meant for them. There are millions of children who really need parents. Perhaps one of those children belogs with your friend.
I am so sorry that all of you are in this painful (for at least one party no matter the outcome) situation and hope that you all find the best solution. I hope that if you decide to get your daughter that she is returned to you easily and know that there is another child waiting for your friend. I hope with all my heart that they find each other very, very soon. Best wishes and hopes that you all have happy and healthy futures with your families and that your friendship is strong enough to survive this and any other storms the two of you face together.
Mother of 4, First Mother, Former GAL