Want to text from Canada to USA and back?

Okay, here's the situation.

I have a friend in the USA. I live in Canada. We want to text eachother for free. I have an iPhone 4, she has a T-Mobile Prism. We both have unlimited texting. She does not have Data, I have 6 GB of Data (so using data for me in order to text her will be no problemo!)

What can we do in order to text eachother for free?

I thought that if I download one of those free texting apps, I could text her for free, but then I thought she may be charged long-distance to text me back at the texting app.

So, what can we do?

Added (1). She has no Appstore on her phone. And I'm the Canadian friend xD

There's this great app called WhatsApp, it's free to send messeges (It's 99 cents on the iPhone though… ) and I think your Canadian friend can get it to (for free) if she has an App Store on her phone.