After I switched from the iPhone to the HTC One, my iPhone and computer kept receiving text messages from friends. Even though I would text them over SMS with the HTC, their response kept coming to iMessage. My wife had the same problem. She thought all her friends were just ignoring her.
After some googling, we figured out the steps necessary to resolve this. First, the person that still uses iOS needs to disable iMessage in their settings.
Go to Settings -> Message then toggle iMessage to off:
Next, the person using iOS should delete all their threads that used iMessage. Simply go to the list view of the messages and swipe to reveal the delete button:
Next, send an SMS to the friend using iOS and then have them respond. This establishes an SMS message thread. The iOS user can now enable iMessages in their settings.
These instructions are all over the place. This is just the sequence that seemed to work best for us. Let me know if there is an easier / quicker way.