IMessage won't change to text message?

I have an iPhone. My aunt had an iPhone but recently switched to an android. When I go to text her, my phone still thinks she has an iPhone and sends it as an iMessage. How do I make my phone "realize" that the message needs to be a text message not an iMessage?

If it doesn't automatically change you should be able to double click the imessage once you've sent it and click "send as a text message"