Well, if she has the serial key then just tell her to borrow the cd from her friend. Thats not really illegal. Just lend her your xp cd, or tell her to ask around I'm sure one of her friends should have a xp cd.
You cannot just create one unfortunately. You'd have to borrow one off somebody, or download one from the Internet and use the legal Serial Key. The use of the software without license is the illegal thing, not the downloading.
Wait! I need to find the thread. There is a program that you install and it tells you the cd key of your windows copy. Then you can just get another copy off a friend and install it, and use the cd key tht the program gave you.