That's a tough one. With both eyes open, when she focuses on the target, she will "see" two arrows. Her brain will choose the arrow her left eye sees and use that to "aim." That won't be the arrow that is lined up under her right eye. As a kid I actually tried drawing across my face, to my left eye, which didn't work for me.
If she's never shot before, just get her shooting. She might figure it out pretty quick herself. Too much worrying about every little detail can suck the fun out of it. On the other hand, not addressing it may lead to bad habits.
Kyle