Iphone/Adroid texting issue, Texts arnt showing up?

Hi, for a while now my friend and i have been having trouble texting each other. First off I have to add that my friend is from a different state so i don't know if thats a factor but anyways. I have an iPhone and she has an Android. I'm able to text the rest of my contacts perfectly fine, and apparently she can text hers fine also. It's only us two that are having issues. My texts arnt showing up on her phone ( Even though the iPhone says its been sent ) and i'd wait hours for a response until way late in the night. There was another time where i couldnt see her responses but yet she could see all of her other contacts texts. I went to the apple store and they said my phone was working just fine, but somehow she still isn't getting my texts. It makes the swish sound, and no red "!" are showing up so i know its been sent. I tried shutting my phone off and on, still nothing. Who's phone is glitching out? Mine or hers? And what can I do to fix this? It's been really stressful not being able to keep in touch since we live so far away. ( of course we could call and thats fine too. But still i would love to get this texting issue resolved as soon as possible. )

For the record i have iMessage switched off, and MMS messaging on