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?
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
The amount of milk breasts produce depends on the demand. If demand increases, then 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.