I'm a bit confused - I thought you guys had all signed a lease together, and that you wrote one big rent check each month to cover everyone.
If you're writing separate checks, your landlord should demand that any further checks from her be cashier's checks or money orders. This is pretty standard... part of the rental agreement at my last apartment stated that if a rent check ever bounced, you had to pay by cashier's check from then on (until you moved out). If you're writing one big check, whoever writes the check should ask her for a cashier's check.
You may want to double-check the terms of your lease... you may have to renegotiate it once Daisy is gone. You will probably also have to cover what was her share of the rent until you find a new housemate.
On the up side, now is a good time to find someone - no one wants to move mid-semester.
Who decides who moves in? Is it up to you, your landlord, or both?
Good luck - I hope you get a better housemate this time!
