I suspect your friend does not have an iPhone. iMessage can only be used between Apple's iDevices.
However, see the last two links.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
My Facetime Doesn't Ring
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Send SMS from iPad
That's the thing, he does have an iPhone. He has an iPhone 4 just like me, so that is how I am able to iMessage him on my iPhone. It is just, when I try to iMessage using my iPad it says he doesn't have iMessage set up. It is the same when I try to message my other friends who are using iPhone on my iPad.
I am also having problems with iMessage for mini-iPad that I just purchased over 2 weeks ago. Just spoke to verizon tech support who couldn't fix the problem and recommended I download iMessage for iPad which I just did.
The new iMessage for iPad Plus (paid $1.99 for it) doesn't even come up on my ipad...half the screen is black and other half is green. The original iMessage application that came with the mini ipad keeps telling me that I have a connection problem when I try to signon with my appleid.
Can someone out there help me with this/
My mini-ipad is up-to-date with latest IOS 6 version; restrictions are off and cellular data is on. My LTE connection shows 2 bars and I do have WI-FI in my home that I've had on to try to get iMessage going...still no luck.
It could be that your friend has not turned ON his iMessage on his iPhone.
I can send my son messages on my iPhone and I think this is because it is being sent as TEXT messages, he does not show up on my iPad3 iMessage screen but other friends with iPhones do.
Ask your friend to go into SETTINGS > MESSAGES and make sure iMessage is turned on.
Then look for SEND & RECEIVE and imput the phone and/or email he wants to use.
Hope this helps.