#7 - June 3, 2010, 7:08 p.m.
Each region has its own unique set of realms, Cryi. This means that North American realms are completely separate from European realms. In order to access North Americans realms, you need to have a North American WoW account, and in order to access European realms, you need to have a European WoW account, and so on.
I'm guessing that your friend set up a European account instead of a North American account, and that's why the two of you are unable to play together right now. All your friend needs to do, though, is to set up a North American trial account—a process which is quick and easy. Just refer your friend to this link:
https://us.battle.net/account/creation/wow/signup/
Have your friend make sure that the URL on her end reads "US" and not "EU." Have her complete the account set up process and she should be good to go.
You might also want to consider referring your friend a trial account through our Recruit-A-Friend program. You can learn more about that program and the process for recruiting here:
Hope this helps! Let me know if you have any further questions, though. :)