Yes. It wont be part of your free texts package (if you have that) as you're texting a foreign mobile. I think it costs about 30p per text. I made the same mistake when my friend moved abroad!!
my brother lives in Oz and i now have as part of my package, they call it a "bundle" free texts to oz, if you are texting her lots it might be worth doing it.
Text using Skype.....it will be very cheap
Text using Skype.....it will be very cheap
Or just do video call on skype. Take she has a webcam? Then its all freeeeeee today!