Is it a coworker you know well and trust? Or is it simply a coworker of age that can be sketch and is scamming you out of your beer, money, and then some?
If it's someone you deem trustworthy and think that this actually happened, pay half. It's reasonable to pay half because he was going to the store on your behalf, but he did fuck up.
If it's someone who is kind of sketchy and you only asked them to buy it because they were of age, don't give them shit and find a new guy to make your runs. You'll never win with a sketch ball... they will fuck you over eventually if you try to trust them repeatedly.