Why Is My iMessage Sent As Regular Sms. Here's The Answer.
Make sure Send as SMS is turned on in your Messages app settings by opening the Settings app and tapping Messages. If you scroll down you’ll see an option that says, “Send as SMS” with a switch to it’s right. You’ll know that switch is on when it’s green and the slider is positioned to the right. If your friends don’t all have it turned on, they may not all receive group chat messages.
However, according to Apple, this shouldn’t be happening. Either the entire group will be iMessage or it will be Text message. Perhaps you could add your friends email address that she uses for iMessage to her contact information? It may help sync it up and fix this issue. According to Apple support, if any person in the group cannot use iMessage, then the entire group message will be text, for everyone.