A plain old 3 foot stick would work as well as any of the over-the-top stuff posted. It shouldn't draw much negative attention, and she'd actually be likely to carry it.
Get a 3/4" dowel from your local hardware store, along with a decorative ball end for it. (curtain rod accessory)

I'd tell my daughter to walk a different route, and consider reporting the neighbor to Animal Control (if you have ordinances regarding leashing animals).

Ok
Once upon a time, I rode bikes everywhere. Of course when u are 12 years old that’s what u did.

There was this St. Bernard, now I’m not sure if he wanted to eat us, or just wreak us, he would run full tilt at us as we went by.

So, we made what we called phasers.  Take a visine bottle, empty it. Fill with bleach.  Aim for eyes, when u hit him he will welp, go lay down and rub his eyes for about 30 mins.  Bonus is you will bleach his fur with stripes.

Impact weapons for a dog put you too close to it, and I’m sorry, a 16 year old girl just ain’t gonna hurt that dog enough.

The phasers will make the dog go, he will want nothing to do with you.  Now the ST Bernard never learned.  It always came after us.  And he had alotta stripes to show for it.  Hitting a dog in the eyes pumping the pedals is a learned skill??

Just what I’d do, and did

Also, in today’s world it could be a good defensive tool.  Not many people would suspect that package to deliver that kinda hate

Pepper spray comes in different formulations.  The kind we used when I was a LEO could blow back in a hard enough wind.  It doesn't take much blown back in your eyes to put you at a big disadvantage.  That said, there's more than one kind.  What she needs is the gel kind that shoots in a stream instead of a fog.  

A collapsible Baton is great but it requires some training as I'm sure you already know.  

Kimber makes a little handheld "squirt gun" sort of thing that is about perfect for what you're wanting in my opinion.  In your situation it's probably what I'd give my child.

One of the Cold Steel war hammers ought to do the trick....

Damn, so its true...? LE does just want to shoot your dog.
Or your one kick ass mailman