IMessage working for all contacts but one?

I have an iPhone 4 running on iOS 6.1.2 and my friend got the iPhone 5 running on iOS 6.1.2 yesterday. She iMessaged me but it was through an email and when she sent an SMS, it was from her actual phone number and a separate message opened up.

Today she iMessaged me but when I replied with iMessage, it sent but like 2 minutes later, it said not delivered. It does the same thing when I try to send pictures too.

I already tried turning iMessage on and off and restarting my phone. I'm also using Wi-Fi. Any suggestions?

Maybe it's your friend. She could've maybe turned off iMessage, or she had a certain phone plan?

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