A large, established website made the decision to change all urls from HTTP to HTTPS. They did all the right things with 301 redirects and canonicals. But now they want to return to HTTP. So the site would go from HTTP->HTTPS->HTTP in a matter of two weeks.
This sounds like a disaster to me. I don't want to get into why they went to HTTPS and why they want to go back, as its a different discussion. I am interested in reporting the likely effect on google search results to them, and understanding any mitigation if they do proceed.