Why are my iMessages showing up as texts for certain iPhone users?

Okay, so I have an iPhone 4S and earlier today, I tried to text one of my friends earlier who has an iPhone 4S and iMessage wasn't registering when I entered their number, and the texts would only send as text messages. I can send iMessages to all of my other friends who have iPhones without any problems. This friend is the only person it's not working for. I'm not sure what the problem is.

It could be because the person is not connected to Wi-Fi or 3G

It might be that, that person has their internet off

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