First off, she wasn't hacked. Hacking involves gaining access to an account. No one gained access to her account, they just downloaded the pics that they could see (easy to do) and made a new Facebook account. Then they tried to friend all of her friends.
Why do they do this?
Sometimes it's a ruse to try to raise money. Sometimes it's to troll the owner of the original account (usually accounts of people who are well known), and sometimes... Well, if I had to take a wild guess, some people are so desperate to be someone other than themselves, they have to steal other people's identities.
It's the same reason why someone would steal baby pics and come up with this narrative on how wonderful motherhood is... complete with a detailed birth story.
People like this need some serious mental help.
What do do when this happens to you?
First, if you are receiving a friend's request from someone that you thought you were already friends with, check your friends list. If they're not there, it is entirely possible that either Facebook glitched and removed your friend from your list. This has happened to me recently when I realized I didn't see a post from someone I knew for a while.
If they are in your friend's list, message them. Don't accept or deny the request just yet. If they message you back from the known account, let them know about the duplicate.
It's entirely possible that they got locked out of their account and started a new one. However, Facebook has made it easy to get back in while keeping it somewhat secure. It's not likely. There might be some questions you could ask that you know that only the two of you would know. In the end it will be your call whether or not to accept the new friend request.
Finally, if it happens to you, report the duplicate page. Your friends could report it, but for the most part, they really can only report the friend's request as spam. The only way to 100% prevent something like this is to not be on Facebook at all.
For more info on this, check out these links:
Snopes' take: http://bit.ly/2odZ9zH
Hoax-Slayer's Take: http://bit.ly/2o1MzRN
Digitally Kidnapped: https://yhoo.it/2nIbtE4 and http://bit.ly/2o1VtyJ