I can think of so many ways of answering him...
1. Tell that to Thomas Cromwell. He did a grand job of cropping Catholicism out of the national picture. And it involved the destruction of many more statues (and better statues) than we are thinking of removing now.
2. Tell that to any historian. Photoshopping the past is what they do. If they weren't continually revising the historical narrative they'd be out of a job.
3. Tell that to the next developer who comes along with a big wodge of money for Conservative Party funds on the unspoken proviso that he gets to develop some tasty piece of real estate. Explain how he can't be permitted to clear away any historical building or artefact because that would be "photoshopping history".
4. Tell that to Winston Churchill. "You can't bomb German cities because that might involves destroying statues- and that would be photoshopping history."
5. All history is photoshopped- and none more so than the version of British history I absorbed as a child. Another way of putting this is "all history is propaganda." If you want to perpetuate the propaganda put out by late 19th century elites you can, but haven't you noticed how badly stained and eroded it is?
6. Getting rid of a statue isn't photoshopping. It's adapting the urban space to make it a more comfortable environment for citizens in the 21st century. It's part of a never-ending process.
7. Men like Colston and Milligan aren't being cut from the picture. If anything their role is being highlighted.
8. The reductio ad absurdum is that nothing we've inherited from the past can ever be changed or set aside. "Hello Boris, why are you walking down Whitehall wearing nothing but a soiled nappy?" "Because this is the first outfit I ever wore- and changing it would be photoshopping history."
These responses are of varying merit. Some of them are almost as silly and glib as the statement they're countering.