It is troubling that a manager would ask a future employee to work before their start date. While I understand that a good start with the new employer is desired, it seems that a polite, but firm "No" is in order. It seems to me that an email reply something like the following is appropriate:
Dear [future boss]:
While it is flattering that you think enough of me to trust me with this work before my start date, I cannot work on this task at this time. While I understand that you need the task done tomorrow, I am still working for [current employer]. As such, I am sure you understand that my current obligation is with them. I will be happy to provide the same level of effort and dedication when I come to work for you.
Sincerely,
[your friend's name]
As I said above, it's troubling that the new boss would ask for such a thing. Although unreasonable, I would fear reprecussions from something such as this, since the boss has already shown him/herself to be unreasonable. Perhaps a cc to the future boss' boss would be good here, if that person's email address is known. Unfortunately, your friend may want to resume her job search and/or inquire about staying on with her current employer.