That's the hardest part of Korra right there: she can do very little in this situation. It frustrates her as much as it does Tenzin, however Tenzin is receiving up-to-date information whenever he can while Korra is just hearing second-hand information, and what little Bolin can tell her without risking his job. It couldn't hurt to tell Korra, even if the 'good' it would do is simply keeping her working with the police. She's a rogue, and this disease is attacking benders. If for any reason Korra believes Lin will not allow her to know anything, it may, eventually, urge her into trying to find things on her own.
That puts Bolin in an uncomfortable spot, no?