Just wondering how much more you need to hear before talking to the mare owner. If she reads CoTH, it may be an interesting conversation already.
It seems to me that you have decided to pay the bill if required because you want the foal. So if that’s how the mare owner interprets the contract, you will presumably say that you wish you had known how such an occurrence would be handled but that you love the foal and will pay.
Perhaps you and the mare owner will discuss what to do if there’s another unplanned veterinary call while the foal is still on the mare. Or maybe not.
I wish you very good luck and most importantly, a healthy, happy foal that grows into a lovely horse.