Friend with Android phone and Mac computer defaults to iMessage in Messages
This is an old thread, but I found it while trying to solve this myself...

The only way I have found to work around this is:


Open iMessage
Start typing the contact's name
When the iPhone displays the contact you want, tap the ">" icon on right of the name.
The iPhone will drop down and show all of the contact options.
Tap the contact's phone number to select the delivery method.
Carry on as you normally would typing and sending the message.