Rowena Ravenclaw was shocked and upset about her daughter's actions, but she hid the loss of her diadem from everyone. However, Rowena did ask someone for help: the Bloody Baron.

If she’d told a Ravenclaw student, this would be somewhat understandable. But Riddle was a Slytherin, and as such, her decision to tell him about the diadem makes no sense whatsoever.