Why are messages being sent as SMS with one contact?

Both me and a friend have iPhone and until 5 days ago our texts were blue and now they’re green and being sent as SMS. Neither one of us made any changes to our setting. Both of us are able to iMessage with other iPhone just between us not goes through SMS vs iMessage. Makes no sense.

Both of us have turned imessaging and FaceTime off, phone off, wifi off, then all back on without any luck of it resetting/fixing the glitch. Neither one of us can figure out whats wrong.