RESPA doesn't explain escrow very well, so your confusion is understandable. Complete an analysis for the upcoming year with the higher taxes (which you probably already did to arrive at the surplus). Add the surplus to the beginning balance and determine if this will be sufficient to meet the increased taxes with the same loan/escrow payments.
You can set her inflow-payments at whatever you need to make the outflow-payments. If she voluntarily wants to allow you take her surplus or adjust her payments, put it in writing (ensuring that it is clear that this is voluntary) and have her sign it.
I discuss voluntary payments and provide a sample disclosure in the BOL Webinar I taught in March 2010. You can find more info here:http://www.bankersonline.com/bankerstore/index.php?main_page=product_info&products_id=971