my sister bought a Prada purse at the thrift store for $1?

Being that the damage isn't a factory defect, you could expect to pay $100, or even much more, depending on the severity of the damage. The only way to know for sure would be to take it to a Prada boutique and show it to them. They would almost certainly have to send it back to Italy (at your cost). Sounds drastic, but any problem that they can't fix in-house goes back to Italy.

You should also be aware that not all damage can be repaired. A scuff on a buckle or a loose stitch on the strap can be fixed; a tear in the exterior fabric would amount to reconstructing the bag, in which case you're better off buying a new bag.

For factory defects, it's free. I found an errant stitch on one of my Prada purses, and they offered to send it back to Italy for repairs (for a simple stitch!) if they couldn't fix it right there in the store. Luckily they fixed it in-house for free. Not quite equivalent to your sister's predicament, but the take home message is that Prada is very good about customer service. They will do what they can to make you happy, but they can only do so much if the damage is severe.

Edit:

I forgot to add, if your sister wants to go the purely economical route, she could try getting an estimate from a luggage repair shop. It won't be genuine replacement materials, but since the bag only cost $1, your sister wouldn't have to worry about devaluation. Just another option.

Its fake

It was prob fake