I think you should go with the advice that she needs to be told about her adoptive dad not being her biological father and eventually that her biological dad has contacted you....
People change, they grow up, they have life experiences that make them into different people. Get to know the man now. I think getting to know what is going on in the dad's life now might be what makes the difference in how you feel. What if he has worked at the same job for the last 5 years, owns a home, volunteers at church, has a decent life??? What if he is a decent man now and you bias her against him? On the other hand what if he's worse than ever? You can't make an informed judgement on stuff from 6-10 years in the past.
I did a lot of research on miscarraige and when parents feel the connection to their child. For mom's, they start becoming a mom the first time they pick up a baby doll and play mommy, so miscarriage effects them differently, they mourn the lost child immed. For men, feeling like a parent comes the first time they hold their baby and feel that physical connection, so when they loose a first child to miscarriage they don't understand the loss the mom feels. They tend to go through the mourning process after they have that second child and they bond with them, they finally realize they lost a child and go through the whole mourning process. He may be feeling that right now and not know how to talk to you about his feelings.
So, now that her biological father has a child he feels the loss of his first child. I understand your feelings but I also think if he has grown up and made changes in his life she will resent you for the rest of her life when she finds out you kept him from her. If he is the same as when she was little then by all means protect her. But she still is going to resent you because you kept him from her. She now has a brother that deserves to know his sister and his sister deserves to know him. Maybe later though, when she's old enough to understand the situation.