IPhone sends message as texts instead of iMessage?

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?

Go to your friend's contact, click Edit, and right next to her iPhone number, it should say iPhone. If it says Mobile or House, change it to iPhone by tapping on the option and changing it. In order to use iMessage, you need either 4G, wi-fi, or LTE.

I assume the problem is that neither of you have the phone number set to be used as a way to reach someone via iMessage. Check the Messages settings & confirm that to be the case & that should solve the problem.
As it stands it would only be the email address for the Apple ID.