Breastfeeding someone else's kid??

In the perfect scenario, let's say you're watching someone else's child for a few hours or more. This child is between 2 and 6 months old. You also have a child that's your own of the same age. The baby you're watching starts to cry because they're hungry and you realize the parents didn't leave any formula or already made bottles. You also know this child is breastfed. You can't get in contact with the parents fast enough and the baby is getting frantic and ready to be fed. You've been beast feeding your child so you have plenty of milk to spare for this baby. What would you do in this situation? pump and give the kid a bottle? Breastfeed the baby? Let the baby by hungry?