I'm having problem with sending texts to my friend. She is getting the messages, but severely delayed, sometimes days late. I have no idea what is causing this problem. Can anyone enlighten me?

Things to know:

I am with O2, she is with Vodafone.
The delay is only when I am sending messages. When she sends me messages, I receive them as I should, near instantly.
Both of our phones are Samsung, but I'm not sure which models. Hopefully that isn't too important.

Can anyone explain the delay?