My iMessage not working on specific contact?

So iMessage works on all my other friends with iPhones but one day at school I turned iMessage off because the messages weren't sending and I used regular text messages. Now that person I was texting only gets regular text messages from me but other iPhone friends can still get iMessages. My friend also has an iPhone but why can't I turn iMessage back on for that contact?

Maybe your friend turned off Imessage

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