IMessage Is Green Instead Of Blue?

I have connection. My Wi-Fi is on and working. IMessage is working regular for everyone but my one friend who also has Wi-Fi and an iPhone. IMessage usually works. But now it doesn't. My send as SMS is off so don't say that. I've tried a lot of answers such as rebooting my phone. IMessage works for everyone but my one friend. What's the problem? And how can I fix it as soon as possible? Please give me an answer that has worked for you. By the way I have an iPhone 4S.

It may not be your phone it may be your friends for iMessage to work both of you must have strong Wi-Fi if not it well send as an sms

The problem isn't your phone… It's your friends.

He should turn iMessage off WHILE ON CELLULAR, let it de-register, then turn it back on and let it register.