Possibly the best thread ever.
I think you should go round, all serious, and insist the only solution is for her to install teeny tiny litter trays on her windowsill. You should insist
on helping her fit them, of course...Insist that all your
butterflies are of course house-trained, so any that continue to poop on her window are clearly stray or feral butterflies. It if therefore her duty to contact the Butterfly Warden and have them all rounded up.
I also love the idea of butterflies on tiny collars and leashes!