If the police think they have probable cause
to open the package, based on a package sent to your house that you didn't get, make them get the warrant. Don't consent to the fishing expedition.
They WILL get one, but you don't need to make it easy for them. If there are drugs in there they are not going to make it easy for her, that's for sure.
Can they arrest her if they open the box and drugs are in there? Well yes, they could, and they would do that whether or not she consented to their opening the mail. But given the fact that she'd never had possession of ths, all by itself this wouldn't be much of a case.
If they can get a warrant, they might get one for her apartment too. So it goes without saying that if there's anything in her place that would link her with drug use, she should do some basic housecleaning while she still can.
If she doesn't already have or know of a criminal
lawyer, this would be the time she would also want to get some local names. And she should remember that anything she says to the police about this package can be used against her. Even things that she thinks might be harmless can be twisted to hurt her. She has the right to remain silent about all of that and she should exercise that.