What is the ultimate gift for someone who's boss is the worst?

Give the gift of being a good listener if she desires to talk about it.  If not, keep your material things.. it would not help the situation.

Take her out for a coffee and give her the chance to talk about it. Make it clear you're not in a hurry.

Gifts are not the answer here.

A gift won't help her.  What she needs is someone she can talk to.  

Need more information from you.  How much do you want to spend, are you male or female.  You can't solve her problems in general or with her boss, but you can make her feel better even just temporarily.

A session with a career counselor.

Wouldn't it be great if you could solve other people's problems with gifts?  But that isn't true in the real world.  Maybe you could share job postings in your department so that maybe she can runaway from the boss.

you could get her some candy

If you leave the company, put a good word in for her with your new employer. Maybe she can come with you to a better situation.