IPhone sends message as texts instead of iMessage? - 1

Ok so my friend and I text a lot and I assumed she didn't have an iPhone because every time I message her, it sends as a text. Well during our break at work, I saw that she had an iPhone. We tested it out again and I sent her a message and sure enough it did it again and sent as a text. She then sent me one and it did the same thing. She is the only iPhone user that I talk to that I have this problem with. Everyone else in my contacts gets my iMessage, and same with her--it's only me who she has this problem with. Is there any way to fix this?

Remove her completely from your phone contacts and then add her again and tell her to do the same thing.