A friend of mine is starting a new job in a few weeks. The contract is already signed.

The manager just send her an email in the morning asking her to do some work until next day. 

She currently has a another job and the manager knows this. It's a lot of work - it will probably take a whole day. 

She doesn't want to start on bad terms by rejecting the request. On the other hand she won't get paid for this and would need to work the whole night. How should one react in such a situation?