yes, the rule is that the credits are for your use only...However, my MIL joined us at the last minute during our December 2008 trip. We had DDP, she did not. We used our dining credits to pay for her meal three different times. Each time, I asked, and each time the waitstaff was very understanding saying "no problem"....it did take them a while to prepare our bill though...but I didnt' mind.
I know we did this at Askershus and Tusker House (breakfast), I cannot remember the other restaurant.
So, I would not plan on it as each waitperson is different, but always ask!!