Hello
I apologize for the late reply. In most cases an issue like this is caused by iMessage settings on the iPhone, it's possible that it;s not set up to send as SMS to non-iMessage (Apple) devices. Please ask your friend to check under Settings - Messaging and make sure "send as SMS" option is enabled, then restart his phone and try again. He could try turning iMessage off just to test if it's the issue.
If that doesn't help, there might be an internal issue with sms routing, you will need to contact AT&T tech support to check on that, the number is 1-800-331-0500.
Thank you,
Dmitriy
