You can get her password by 3 way. I have listed it below.
1) By using yahoo password recovery tool.
2) By using Keyloggers.
3) By using I cannot access my account link in yahoo login page (for this you should know the secret question answer)
My favorite one is keylogger. All the keystrokes … Read more
If some one hacked her ID , definitely they will change the basic information of that account, try recovery for some time.. If it doesnt work hire a hacker to open your account.