sms/mms text vs imessage?
how do I designate a contact to send only sms/mms text and not imessage even when the contact has an iphone?
iPhone 5
You don't, if you're sending to an iphone user, it will send it as imessage. The only time it will try to send it as a SMS is when the recipient is in an area with a weak wifi/cellular data signal.
Send messages with your iPhone, iPad, or iPod touch - Apple Support