the down side of a photographic memory is that it tends to fade as the person ages.
Happened to an engineer I knew
That makes sense.
The actor Ray Collins was said to be able to read a script just once and have it memorized. He mentioned in an interview that he could no longer do it as he aged.
The claim is that "eidetic memory is typically found only in young children, as it is virtually nonexistent in adults", and there is great skepticism about the existence of "photographic memory".
HOWEVER! I can report that until my middle years in college, I had a semi-eidetic memory. It worked pretty much as advertised on certain things that I was interested in [books, equations and mathematical stuff], but not on other things that I was disinterested in.
It was quite useful in tests - I didn't have to write the answers on my arm - I could just pull the page up in my mind and look at it.
The "fading" that you mentioned actually happened to me to some extent, Chickensoup.
However, my diagnosis of the fading is that my mind became "full" [too many info photos], without the room to effortlessly store the info as in the past. This "fullness" has not disabled my mind in any way, though. I'm just aware that I can't store info exactly like I did when I was younger.
I still can use it on occasion, just as long as I don't try to rely on it to store too much new information.
I suspect that your engineer may not have been able to analyse what was going wrong with his beautiful tool.
https://en.wikipedia.org/wiki/Eidetic_memory
Eidetic memory (eye-DET-ik; sometimes called photographic memory) is an ability to recall images from memory vividly after only a few instances of exposure, with high precision for a brief time after exposure, without using a mnemonic device.
Although the terms eidetic memory and photographic memory are popularly used interchangeably, they are also distinguished, with eidetic memory referring to the ability to view memories like photographs for a few minutes, and photographic memory referring to the ability to recall pages of text or numbers, or similar, in great detail.
When the concepts are distinguished, eidetic memory is reported to occur in a small number of children and as something generally not found in adults, while true photographic memory has never been demonstrated to exist.