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.