I think deep down we all know the decision that we will make its just sometimes hard to make it. Dont second guess yourself do what you want.
My thought is if she already fits in and she seems happy and you love her and want her then keep her
If you think she would be happier being only the second dog in a household that let your wonderful friend have her. Does your friend plan on getting more? She might end up in a house full of dogs anyway....
Dont let anyway sway your decision making. You will regret it.