Please answer seriously, do not waste your time and mine, and no sarcasm..
Given that YOU are the one asking for free advice, do try not to boss the volunteers around with your crappy attitude. There's a reason your post has been sitting untouched, and that's the reason.

Generally, if she receives the receipt before her birthday, they will look at it as if she is under 21. If she receives it after, then after.

She should check with her nearest USCIS field office to confirm.