Could be a lot of things. If the 10 and 20 are still on then it is most likely something to do with the way she is seeing the 30 yard pin, or she is peeking, punching, or dropping her bow arm or someother flaw in technique.
Make sure she is still looking through the peep sight at 30. Sometimes when a new shooter is raising the bow up for a longer shot they inadvertanly look under the peep in the gap below the peep itself. They don't realize it. That is often an indicator that the peep is too high to begin with.
Have her draw the bow with her eyes closed, find her anchor, settle in, then open her eyes and look at the front pins. She will need to immediately focus on the center pin and do not move her anchor or her head. After she has acquired the pin on the target ask her if it is centered in the peep. If not adjust the peep to her. Do NOT have her move her head to see through the peep. Bring the peep to her, do not bring her to the peep.
This may NOT be her problem but it is a good place to start. Next watch her shoot and see if she peeks or drops her bow. The arrow rest may actually striking the arrow as it passes through, that can cause this problem. The grip suggestions are something to try, but I think you would see some effect at 10 and 20 if that were it. Same with the arrow rest or the fletchings. I'm not saying that it can't be these things but I would look to her shooting form first. 10, 20 are great and things fall to pieces at 30--???? Sounds like she is doing something different at the longer range. You may be duplicating her mistake because the bow is the wrong draw length for you and if the peep has a large gap on top or on the bottom in the string you may be doing the same thing as her.