I have a few suggestions that might help.
But first here's a short background information.
This is what i think happen...
When you "swith on" iMessage on your iphone.
By default it automatically send your numbers to the apple server (similar to registering your email in the "receive at"), which register your number to send and recieved iMessage.
note: iMessage only work if you have Wifi or Data (3G) switch on. If not, it will send as a normal SMS (unless you have "Send as SMS" switch off)
So this is what you could try:
1. Get your friend to pop his SIM to an iphone then switch off iMessage.
2. Instead of continuing with the previous messages, create a new message and enter the numbers in manualy.
3. Try turning off the iMessage, then restart the phone before turning it back on.
hope one of them help
for more information on how this work, you can find it here:
http://www.ilounge.com/index.php/articles/comments/instant-expert-secrets-featur es-of-ios-5.0/P3