Usually shows don't offer refunds. But in Wicked's case, it is popular and usually a sellout, so there is an excellent chance they'll be able to resell that ticket.
The other case they'll offer a refund is if someone's name is billed above the title of the show, and they are absent that day.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611