Bicycle Mechanics - matching BB Spindles and Cranksets to avoid rounded out loosened crankarms I am

I am having a terrible time getting my wife setup on a 6-speed city beater.
Her left crankarm keeps loosening off and falling off. Which makes her angry and me look like a dope for not being able to get this working reliably.

(and she even did this on a modern 10-speed Record crankset up. is it possible that someone's pedaling style can cause crank boits to loosen?)

Originally this is a Vitus frame with DT shifting Shimano 105 bits. I don't remember the original crankset I ran it on for years and years before passing it on to Julie.

The latest crankarm to detach (and undoubtedly be rendered useless since the alloy square slot would likely be rounded out.) was Shimano 600.

and then a Stonglight left arm did the same.

I am ready to start again but I don't want to destroy another crankarm, especially a nice one.
I have a box with about 30 BB sets and a whack of cranksets. (though I would rather not send her out on the street with Nuovo Record gear. )

I was told that the spindle shape has to match the crankarm hole, to ensure the snuggest fit.
Stick with Japenese BB to Japanese cranks? French to French? etc.?
But how the heck to I ensure that? especially since the country of origin of many of my BB are not marked (the cheaper ones in particular.)

Many thanks

Almost all modern spindles share the same taper, but the starting width at the end varies. That means that while the taper fit is OK, mis-matched cranks and spindles will have different depths of engagement. On some the spindle will come all the way through the arm and cannot be tightened, on others the opposite where the spindle will only enter the crank to less than half the depth.

Ideally you want the spindle to reach to within 2-3mm of the end of the taper, allowing maximum contact area, while leaving room to be drawn on as they tighten.

If you're having constant issues of loose cranks on well matched parts, I suspect the mechanic and rider both contributed. Odds are the crank was never properly tightened onto the taper, and continued to be ridden after it began to loosen. It's also possible that the crank was shaved by the forward edge of the spindle during installation. I've seen many spindles with sharp corners at the leading edge, and make it a policy to chamfer all four sides with a grinder or belt sander before using. Spindles from bigger companies rarely need this step and it's done in production.

In summary, get a properly matched spindle and crank, install it to proper tightness, and ask the rider to notify you at the first sign of looseness, evidenced by clicking once or twice per crank revolution.