iPhone won't send text messages

Both of you should check Settings>Messages and make sure that iMessage is activated. Also, you both need to ensure that you are connected to data, either cellular or wi-fi. You may also need to delete any iMessage thread you have between each other, then start up a new one.

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Are both of you using an iPhone? What happens when she attempts to send you a message? The answers to these two questions will help determine how to troubleshoot the issue from there.