IMO it is not over-tightened spokes. I've had something similar, where a rim started cracking radially away from the hole. Adjacent spoke holes showed similar but less damage.
My guess is that it was from a large impact with a road fixture, where the afflicted spoke was at the top of the wheel opposite the impact, so when my weight hit the saddle and the rim was forced up and back by the edge of the surprise-kerb.
So ask the rider if they've hit any big potholes or other hard impacts, and check the opposite side of the rim for dings or dimples. The tyre probably won't show any damage.
That rim is shot and needs replacement. It is not safe to ride.
You could buy a new rim and reuse the hub/spokes. It is always good to build a wheel at least once in your life. Then get it final-trued by a LBS.
Contrary to wagon wheels, spoked wheels are "hangers" under tension, not "compression" spokes to carry the load. A static bike wheel is distributing all of the bike's weight on the top few spokes. Not sure what the distribution is, but the top one is carrying the single most weight, with the horizontal spokes carrying none.
If the wheel is rolling, the dynamic situation will confuse things even more, but the weight of the bike+rider is still hung not held-up.