Schwinn used to do it on some models, but not on lightweight forks. I have seen it on a World, which was a high-quality touring bike in the '50s, and also on a tandem. Can't remember if it was a Paramount or maybe a Town and Country, which was also a high quality hand-made frame, but heavier than a Paramount. Also probably '50s era. I have not seen it on any more recent forks.
I put slight indents in a MTB fork I made in '81. It was really a hybrid "mountain-trials", 26" wheels but made for Observed Trials. I rode over a lot of picnic tables and natural obstacles that size or larger, often coming down on the front wheel which sort of simulates a frontal collision in terms of the forces on the fork, pushing the blades back. Other times I lauched off a dropoff and landed both wheels at the same time, which pushes the fork forward. Although these events were not as frequent as say pedal strokes, which cause high-cycle fatigue, I think a series of crashes and near-crashes might qualify as low-cycle fatigue. Anyway, whatever you call it, this fork withstood it just fine for some number of years, never yielded or cracked. When I sold the bike maybe 10 years later I put a different for in it, so I still have the '81 fork, waiting for "just the right bike" (which may never happen).
That's just one data point, and also the blades I used might be stronger than what your friend used. I used Reynolds Tandem, the oversized "Jack Taylor" blades. In a tandem crown made by Angel Rodriguez, intended for road bikes, but almost wide enough to fit 2.125" balloon tires (54 mm). They did clear without indents but with no mud room, so I gave 'em a skosh more room. If your friend's blades are weaker and/or if his indents are more severe, then it might be crazy to ride it.
Like Andrew said, those slight indents are something I'm willing to do on a bike for me, but I wouldn't do it for a customer.
Last edited by bulgie; 12-19-23 at 01:51 PM.