How to remove red from the photo please?

Hello,
I’d like to share a true story from the late 60s when I was doing an internship with a rural photographer who was an expert at retouching (think Photoshop V0). He worked on 18x24 negatives (cm, mind you) and applied a layer of "matholin" to make the gelatin suitable for holding the lead of his sharpened pencils, which were honed to 3 or 4 cm for maximum flexibility...
One market day, a local came in with a contact print 18x24:
- It's a group wedding photo, and it's the only image we have of my deceased father; can you isolate him?
The negative being archived, it would be easy, so the photographer replied:
- Yes, no problem
- Yes, but can you erase the people around him?
- Possible too; we'll put in a gray background, but it's going to cost you a bit more.
- Yes, but in this photo, Dad is wearing a hat, and he never wore one; can you take that off?
- Uh, yes, but you'll need to tell me his hair color and how he styled it.
And the other one replies:
- But for goodness' sake, once you take the hat off, you’ll see him clearly!

Luca0909's question reminded me of this anecdote

--
I call a cat a cat *
*in French in the text