EDIT - You can ignore me as you probably have a much better read above mine - was beaten to the punch! 
Hi Carly,
The EllisLab admin will be able to answer this better than me but from my knowledge of how it works :
Either you or your friend can purchase ExpressionEngine. If it is the case of your friend then they can download the files and then either give you their login so that you can download the files or they can send the files on to you. Alternatively if they know how to upload the files then they can do this and they will be on the server ready for you to work with.
I think that as long as you (if you are not the paying customer) don’t use the files anywhere else as the files are literally a one site one licence affair then you should be okay. As you are friends as well this makes it even easier, you could do this from their home if that is an option.
As the designer / developer on the site though I believe I am right in saying that if you (or your friend in this case) have purchased the software and are obviously entitled to support in the forums then you (or better your friend) can e-mail sales directly and get you a forum log-in so that you can post any questions you have in the paid technical support forums.
As I said before I’m sure that the admins will answer this better than I can but this is the way I understand it to be. I’m prepared to be corrected though.
Hope that helps and hope it is correct!
Also welcome to the wonderful world of ExpressionEngine. You are going to love it.
Best wishes,
Mark