Every place has some or the other drawbacks city is better now but it's also on self to take care so Just ask her not to visit lonely places..avoid late nights exploring..get location app on mobile while travelling ..
She can travel to varanasi, but she need to be alert, and also its good if she avoids going out alone after dark. Overall its a good place to explore she should go if she wants to.
No think there some dirty around ganga but due to spiritiual values people visit there and no one throws dead bodies and all about perverts you can take percaution first like pepper spray and other things