Texting
my texts to one friend are "undeliverable". I've been texting this friend at this number for years, nothing changed. I can text him on my iPhone but not on my iPad.
iPad (3rd gen) Wi-Fi, iOS 5.1.1
Are you sure that he did not turn iMessages off or sign out of iMessages? Did you turn off text message forwarding on your iPhone in Settings>Messages>Text Message Forwarding?
You can text him on the phone using standard SMS messaging which does not work on an iPad unless you use text message forwarding from your phone.