I agree with the bolded. As the OP is assuming that the girl doesn't know she is adopted at all, it really is a question for those of us who believe that our parents are our biological parents: How would you feel if, out of the blue, you learned from a stranger that your parents really aren't your parents at all?
I have a "cousin" (actually the daughter of my mom's college roommate and best friend) who knows she is adopted by her 3rd mother's husband and believes that her mother's 2nd husband is her bio-dad. What she doesn't know is that her mother and husband #2 adopted her when she was 2 days old, at a time when adoptions were sealed and very hush-hush. My cousin is in her mid-60s and still doesn't know. She probably never will. ALL the relatives who might have been in a position to know (i.e., her mom's and mom's 2nd husband's families, her mom and dad, and my parents) have passed away, and both families were very small with no additional cousins so there really IS nobody left on either sides of the family. (It's really how we came to become "cousins;" our moms were SO close and neither sides of either families are very big, so we are all we had growing up.) ANYHOW... I have good reason to believe that my sister and I are the only ones who know that my cousin was fully adopted by her parents at birth (because my mom told us about 35 years ago), and I feel no compelling reason to tell her. She is a happily married woman, a mother and grandmother, and I don't see any good reason to change this. There is nobody to corroborate it, and, well... it's something that her mother didn't want her to know at the time, for whatever reason. I honestly don't believe it's my information to have had in the first place (NEVER tell my mom anything in confidence, she's always going to tell someone), and don't see what purpose it'd serve to "tell" now. (NOW... I cannot 100% guarantee that she doesn't know and just didn't tell me, but I know that when we were in our mid 20s she didn't know, and her relationship with her parents never seemed to have changed, so I'll assume that she still doesn't.) It's really not something I worry about, but this thread made me remember it.