International messaging is not included in your messaging pacakge. It will be .25/send and .20/receive messages. You can confirm this under the International Services page on this website; here. So it's probably best to keep chatting thru MSN.
If you both have iPhones texting on imessage over 3G, 4G, Or wifi is free to any number or email registered with imessage so you could talk that way if that's possible.