I remember reading this post several days ago so I know this this has not just occured.
I do not believe that frontline can be adequately removed with water. There are special shampoos that remove it and even then I don't know if the shampoos are safe for a rabbit.
This is definitely a problem for poison control.
There is a feefor the call
Unforunately the call should have been made immediately after the rabbit was sprayed..even now it is worth finding out if anything can be done so this rabbit will not suffer and die.
Please keep us up-dated and please tell your friend to call poison control. Watching the rabbit is not enough.