Guest Refund

Hello Everybody

I'm new to Booking.com and have a query regarding guest refunds.  

My guest booked two nights on separate booking references.  She stayed the first night but left the following morning, she wants a refund for the second night, (which has it's own booking reference).  Now here's the rub; she paid a discounted, non-refundable, price.  Because I'm new to this I agreed to refund her.  Booking.com sent a message requesting my agreement to refund but this message has timed out.   Two questions:

How can I refund my guest? 

Can Booking.com re-send the agreement message?

Any and all help gratefully received