I totally agree as well. The mom over-reacted and is probably having a hard time sending her kid to daycare in the first place.
calling her this evening when she has a chance to think about is would help. I like the sibling relationship analogy mentioned above. comments will be said, the will play like best friends, adn they will fight like enemies. Its all part of kids being themselves and expressing every thought that pops in their head!
I do hope she agrees and sends dck back soon, and in the mean time, continue to advertise to fill a second spot. It could take some time to find more so may as well start now.
It would help with the financial part of things thats for sure!

Good luck, and keep us informed!