My friends has imessage enabled on his iphone, but I just can send him a text message, not an imessage. I can send my other contacts an imessage normally.
Delete any message thread you have with that friend. If he is a contact, delete his/her contact card in your contacts. Then add the friend back as a contact. Send a text message. Be sure you have iMessage on and that your friend also does. How do you know your friend has iMessage on? Does he/she iMessage with others?