How do I send someone a normal text if they also use iMessage?

Solution 1:

Choose to send as text message by tapping and holding the message till you see the option to send as text.

Solution 2:

Well, one way that I know you can do it is create a new Text Message, and hit the + icon. Now, when you select your buddy, you should be able to select the phone number or the email address (since that is what your friend is probably using for iMessage). Select the phone number and it should send it to his Windows Phone instead of his iPad.

Solution 3:

Tap and hold the Send button for about 3 seconds and iMessage will switch to text message.