We have a newborn baby. Yesterday, my wife breastfed an older baby of our friends. She is 3 months old. Her mother was at the hospital, so my wife breastfed the baby.
Now my wife has less milk and my newborn stays hungry.
Should I be worried?
The amount of milk breasts produce depends on the demand. If demand increases, the so does production. Just give it a day or two in which you regularly let the child(ren) suck, and the amount produced will increase.
You shouldn't have anything to worry about, since milk production increases to keep up with demand. This is why it's possible for a woman to feed, say, twins. When the baby is no longer breastfeeding, the breasts stop producing milk.
The only problems you might encounter would be extremely short-term, unless your wife has supply issues. If this is the case, a lactation consultant can be of help. (Visits with one are covered by many insurance plans.)
Strategies to ensure a healthy supply include pumping while away from the baby/babies, and massaging to express milk. For more information: Maximizing Milk Production with Hands On Pumping