回复：Thanks so much for the prompt reply. A couple of more questio

I thought AP is needed for her entrance to US next time. Does she need to show AP when she LEAVES US as well?

-Not need, but she need to provide the I-94 to the airlines and thus, USCIS will know she leaves US.

If not, can she just leave and I will mail the AP to her in China once it's approved?

-can not do that. She has to get the AP approval before she leaves.

What's the general guideline for rescheduling appointment? Can it be rescheduled at all?

-Normally you are not allowed to reschedule the interview appointment unless very special situation and need to contact USCIS.