Why are they doing this?
Hackers are expecting some friends of Ms. Binita to respond to her mail and send the asked money. Just respond and see how these people can fool you so convincingly.
How to prevent this?
This can happen to your account also. You should be careful when typing your password on public computers. Always log out from your account while leaving. Change your password from time to time. Make your password strong-text mixed with numbers.
*If any of you know Ms. Binita, please inform her (not by email though)that her yahoo account has become a beggar online and she should do something about it immediately.