Friend left iPhone for Android. My iPhone insists on sending iMessage instead of text. Help

Your 'friend' forgot to de-activate iMessage on her iPhone before she dumped it for a droid.

It doesn't affect what she sends you. What happens is that when you reply, iMessage checks Apple's servers and finds a device active with her phone number, so it tries to send it as an iMessage. iMessage is tied to the phone number as well as an e-mail address.

Tell her to try this:

1. Go to: https://supportprofile.apple.com/MySupportProfile.do
2. Log in if not already logged in.
3. Click on "edit products"
4. Click on the "x" to the right of the product.
5. Click "unregister"

How long before this takes effect?