Well you obviously have a computer. And she has a phone. This worked for me. In fact it still does now and then.
I use
http://www.talk2china.com/english/ Very cheap to use.
You can use one of the online translators. Write your message then copy and paste it into the text box send it to her.
She can reply then you copy and paste her reply back into the translator and read it. It is super simple.
During my very first visit to china, My wife and I actually went to the internet cafe a few times and sat there a couple hours and sent messages to each other and we were sitting right next to each other.
And now we still do the same thing but we use QQ instead of her phone since we both have computers and our desks are right next to each other.
Just a little warning. Be sure and explain to your girl that the translators sometimes make very bad mistakes. And for her to please remember that, because you would never say anything to insult her or hurt her feelings. This will save you some possible misunderstanding. Sometimes the translators are right on and other times they really suck.
Here is a list of the 3 translators I use most of the time.
http://imtranslator.com/default.asphttp://www.systranet.com/http://qq.bur.st/qqtranslator.phpAll 3 of them have different features. And all 3 of them have a little different translation.
When her and I are talking very serious I will have all 3 of them open in 3 separate tabs. Sometimes what she writes will just not look right. So I put it in a different one and it will make more sense.
I do suggest you at least bookmark all of them, and try them all, and very quickly you will figure out which is your favorite.
As far as buying one the Systran is the only one I recommend. It works very well and it is the only one that I was able to find that continued to work with no internet connection.
In fact I have recently bought a Netbook and carry it instead of the besta. It is very handy the net book with 10.1" screen is about twice the size of the Besta. But it is a full working computer and I have found many areas in town with free wiFi.
All this might be more than you need but it has certainly made my life in China much easier. I do not regret any of it.