I just tried something and it worked....
I am the owner of 2 iPads and 1 iPhone 5, all of which are registered in my name with Apple.
One iPad is used by my wife, the other one by myself.
In my Contacts list, I looked up her name, and at the bottom of the details there was a "Face Time" icon, along with the usual "Share Contact" "Send message" icons, etc.
When i touched the Face Time icon, one of her email accounts (different from mine, of course) showed the small face time icon next to it. I clicked that one and it worked.
I tried the same thing from my iPhone and it worked too.
Please note that my wife's iPad is registered with Apple in my name, but the only default e-mail account used on her iPad is hers.
Please let me know if this helps and if it solves your problem.