If your guest is listed in
MDE on a WDW resort reservation, you can order a MB via MDE. The plain ones cost $5, I think, and there are numerous lovely new colors. There are also many different designs available, at various costs discounted from what they'd be at
DisneyStore.com, which is another source for buying them before arrival. There are even some "pre-arrival exclusive" designs available via MDE.
However, if no MB, your guest can get a key card at the front desk on check-in to the resort. I'm not sure whether admission will be on it, but based on recent references to Key to the World cards here on DISboards I think that might be a possibility. If it isn't, they can get an RFID ticket at the turnstile of the first park they visit - just show the confirmation number for the ticket purchase to the CM at the gate. So they might have a key card for the hotel and a separate ticket card for the parks, or they may be combined into a KTTW card, I'm not sure. Hopefully someone will chime in with definitive answer to that question!