imessage

I hope you can help.

i have an iPhone 5s and an iPad air with wi fi.

Until a couple of months ago, my wife had an iPhone too, which were both registered at Apple as being mine.

She now uses an Android phone with the same number.

For some reason, I can send her a text message from my iPhone but the iPad insists on trying to send her an iMessage.

I have deleted and re entered her as a contact.

I have changed her phone number from "iPhone" to "Mobile" on her contact details.

I have reset my iPad.

I have deregistered her phone on Apple website.

I have read loads of forum suggestions and nothing seems to work.

The iPhone is no longer in our possession. To turn off iMessage on there.

Apple should have sorted this longstanding issue by now!

Help please, bewildered and frustrated!