I recommended she call an animal rescue or shelter to find out what we should do, and she wound up getting in touch with a vet. The vet said that she should put the bunny back outside to see if its mom came back to it. [Upon further research I discovered mothers only nurse their babies for about 5 minutes per day, and come back in the morning and evening.] After talking to the vet, she put the bunny back outside, and when we woke up this morning, it was gone. When she took the dog outside, the dog found a nest in the grass, covered up with dog fur and grass - and there were two bunnies inside. She wasn't sure, but thought one of them could have been the one she'd found yesterday.
So, long story short: if you see abandoned or sick-looking wildlife, wait! Call a vet or your local animal shelter first before trying to bring the animal inside or feed it. In a lot of instances, taking in a wild animal might actually make the situation worse - baby birds, for example, will be kicked out of the nest if the mother smells that they've been touched with human hands. This might not be the most scientific site, but the information seems pretty accurate about what to do if you find orphaned rabbits. Our little guy sure was cute, but I hope he was reunited with his mama.