Have your daughter ask her friend to stop copying her. If this does not work, have your daughter tell her teacher and perhaps request a new study partner. If the teacher doesn't believe your daughter, then is the time for you to get involved. That's the basic "chain of command"; however, the teacher should likely be made aware of it anyway because this could be an issue later on and if not handled properly, could come back to your daughter. It would be a bad day if her "friend" turns around and blames her for sharing the answers with her.
I understand that your daughter likely values her friendship rather deeply and may not want to compromise that relationship. For that reason, she may choose to go straight to her teacher rather than confronting her friend about the issue. However, seeing that they are old enough to be taking practice SAT tests, the "friend" should know better than to cheat off of other's answers and might not be the best person to be hanging out with. That being said, this still might not be grounds for breaking off a friendship.
If your daughter is able to choose her own study partner (as opposed to the teacher saying, "Hey, you study with this person..."), she could perhaps quietly pick someone else to study with. If her study partner was assigned to her, then perhaps she can request a new one (as I previously mentioned).
A little bit of personal background on this... During EMT (Emergency Medical Technician) school, I often sat with someone who routinely tried to steal my test answers. I solved this problem by blocking my papers with my arms and shoulders (hunching a certain way so that he couldn't see what I was writing). In his case, I didn't tell on him because I knew that the issue would speak for itself. He was going to have to take a written computer exam issued by the state of Indiana. He would be sitting at a partitioned computer with people around him who could be taking a wide variety of un-related tests. If he was cheating and didn't know anything, he was going to fail the state written.
So that leads me to giving her another possible option. Hide your answers in some way, if she has long hair, flip it to one side so that it blocks the "friend's" view, hide her answers with her arms or shoulders... There are a variety of options here.
Also, when it comes time for the actual test, it is always a good idea to sit so that there is a good chunk of space between each student (my guess is the school will do this anyway). This is also helpful for those of us that like to spread out a little bit during a test.
I hope this is helpful, if you have any questions or concerns, please let me know.