Maybe of interest, about a year back a teen boy died in Bend Oregon at their river wave in a similar accident.
One might ask why can't the person drowning be freed? There are afterall a dozen or more athletes in the river, perhaps more experienced in dealing with its currents than anyone else.
In both cases, all of the other surfers certainly did try, one after the other after the other non-stop. The problem is that water is so darn heavy, and it is pushing so hard and so fast, that even if a strong young athlete can get a hold of the person underwater, or the leash, or whatever the leash is hooked onto, the full force of the river on a submerged body is pulling so hard that it's not possible to dislodge. Unbelievably difficult to hang on in those conditions long enough to disconnect a leash from an ankle. And nobody is carrying a knife to take a slice at the urethane leash.
It would be a terrible way to go, a tragedy for the victims. And must have been really hard on all of their peers desperately trying to save their lives but not able to.