Houttuynia cordata was certainly out of the question for Devereux to brew for Percy.
As for Snape's "proposal," Devereux chose to ignore it outright. However, Snape subsequently offered a relatively sensible arrangement: Percy would stay in the laboratory first, waiting for the lycanthropy symptoms to subside. Devereux would need to brew a standard curse-suppressing potion for him, and then escort Percy to the isolation ward in the Hospital Wing, keeping his appearance hidden from others as much as possible.
Devereux had a vague suspicion that this was probably Dumbledore's idea as well. That "impromptu celebration" earlier might have been partly intended to clear out the others and reduce the risk of Percy being discovered.
Chapter 162