888-447-1249 Hack Attempt HELP
So my 15 year old niece had this pop up on her macbook... she called the number... and she said that they walked her through steps allowing them access to her computer. UGH!!!! Now I have no idea how far they got. She eventually figured out something wasn't right and hung up the phone. She called me and she immediately shut the computer off. I guess my question is what should I do next? Do I need to wipe this machine and start it over? I have it at my house and just turned it on again for the first time (not connected to the internet). Everything looks fine but I'm worried. Any tips on what to do next?
She is on a 13" Macbook Early 2009 (Yosemite 10.10.1)
MacBook Pro with Retina display, OS X El Capitan (10.11.2)