The intialthought would be to make sure that the number the message is going to is in international format (ie +[country code]). I'm not sure whether this would be automatic for a reply, but you should be able to see the number in the message list (I think). If the number is in her contact list, check the formatting. If it isn't, add it and format it correctly, then try again - maybe even with a new thread rather than a reply.
