imessage for some contacts and sms for others

My wife just bought a 4S, my (grown, married) daughter has a 3GS upgraded to iOS 5.  My wife would like to send text messages to my daughter but my daughter does not have SMS on her phone plan, so messages cost $0.25 each.  The question is how can my wife use iMessage to send messages to my daughter so that they are only received when my daughter is on wifi (without incurring SMS charges), but also allow my wife to send SMS messages to all her other friends.

I've read that there is an option to turn off SMS/MMS.  This will let my wife iMessage my daughter, but would block her from texting her other contacts.  Instead of having my wife turn this on and off several times a day, I am looking for something simple that will allow this work a