I'm not very into mobile phones so correct if I'm wrong, but isn't the limit for a SMS message 160 characters? Now if you send a text message longer than that, it actually sends several text messages and thus is counted as several messages on the service provider side, even though it looks like one message on the modern mobile phones.
Now, filling 160 characters is not hard. With an average word length of 5.1 in English that would mean around 30 words per SMS when including whitespace. Even this post, that would be counted as not that long by Slashdot standards, would make up 7 SMS messages giving a count seven times larger than it actually is. Now imagine that you combine this with lots of short messages like "OK" you could easily get a large number of SMS messages even though your communication has really not been that intensive.
Does this explain the bizarre amount of messages this girl has sent? No. Is there a possibility that she has "broadcasted" her messages to all her friends for instance? That would explain a lot.