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]:
> 
> 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 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 before, 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.