Well, I agree it wasn't nice. It's not like a cat can be a blood traitor, even if this cat seemed to like Granger's company.

But Professor Sprout said she can fix the cat, just not right away. So maybe whoever played the prank didn't realise that the cat couldn't be put back to rights straight off?

Professor Lockhart said he's going to help with the potion and then that cat will be fixed even sooner, though.