One of their friends was surprised Jen had alcohol in her system that Jen and Sarah were not drinkers even when they ordered Pizza they ordered Pepsi. It could mean one or both were alcoholics staying sober or they just didnt like to drink or Jen was a closet drinker.
My husband says ( I have to quote him because I do not know this my own self) that some people who drink do not want to pay bar prices so they drink at home and then have very few drinks at a bar. That way they can socializie but it is not as expensive.
Another trick people do, and I have seen this, is rounds are bought for everyone. Conveniently, people are in the bathroom when it is their turn.
So they may have been drinking at home or they did not want to share in the cost of drinks