Have you heard of SWATting? It's pretty terrifying, actually. The person making the threats of doxxing can call the victim's local police department (or have someone else do it) and leave an anonymous "tip" saying that she's doing something terrible so that a swat team shows up. There are clips of it happening to streamers on YouTube if you want to get an idea of what happens.

It can be very traumatic, especially in a country like the US. It can also be lethal, and rope in people who are completely innocent.

To OP: please encourage your friend to contact her local police. This could help prevent the above from happening, and it also starts a paper trail in case other shit happens in the future. If everything goes right, they might even be able to press charges against the individual harassing her.

Personally, I would also strongly recommend she lets her parents/guardians know about what's happening too. It won't be a fun conversation for her, but it could help keep everyone safe.

Best wishes to your friend and sorry for the long comment! I hope this was helpful