How do I disable iMessage for a contact who no longer has an iPhone A friend had an iPhone and when I added her contact I set it her number as "iPhone". She's  recently got an Android device but my iPhone keeps trying to iMessage her, needless to say  she's not getting my messages. I can obviously press and hold each message and "send as SMS" but I'd like a more permanent solution.

How can I make my phone always send messages to her by SMS?
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.