How do I disable iMessage for a contact who no longer has an iPhone

To dissable iMessage on your phone (the one who has changed from iPhone to something else): http://support.apple.com/kb/ts5185

Since it’s not your phone that needs changing, Your friend is the one that needs to take action to Approve the request to de-register their iCloud/AppleID from their phone number - https://selfsolve.apple.com/deregister-imessage

As long as their SMS number is associated with iMessage via their Apple ID, iMessage delivery will be preferred by iOS when sending. If you must send an SMS without them fixing their account, you can turn off iMessage entirely on your iPhone and then send a SMS via your cellular carrier to that contact before turning on iMessage again if you wish to have that for other conversations.

It seems like there are one or two convoluted methods to manually reset a given contact's phone number to stop sending iMessages to them, involving switching the iMessages setting on and off and rebooting and resetting the phones while also deleting the contact and manually adding it again on one of the iPhones. That way seems pretty crazy, and I hope Apple figures out a better way to resolve this issue.

Meanwhile, if you send a message to a non-iMessages user, and the message shows a red exclamation point (showing it didn't send), you can tap on the exclamation point and choose "Send as Text Message". Do this two more times to the same contact, and it seems the Messages app finally catches on that you only want to send text messages to this particular contact.

Double click the text if you have already sent it. Then you will have an option as sending it as a text messages and hopefully it will go through.