I sent my friend an email she thought was 'rude' so she forwarded it to her mum!!?

Hmmm, well, you could send her another email that says something about you know that when you are angry you tend to say things you don't mean. Just say that you appologize for sounding rude to her mom, that was not your intent. You still want to be friends and will talk to her when she is ready. Then leave it at that and stay out of her email! (I know its hard, I know someone's pword too and although it is awful, I went in too--I felt terrible for days.) Good luck, hopefully it will blow over. Oh, and don't say anything about her forwarding it to her mom, she will get mad. Just hope (don't look) she forwards the nice one too.