Sad event.
Sorting out a safe distance from a rotor for wash is an interesting analysis. Doing a measurement of flow from an established hover is straightforward. Add winds, and that becomes a bit more interesting, with T/R interactions. But that isn't the highest flows that can occur from the rotor. Taking a deceleration into a hover if done at heavy weights, and with any tailwind will generally give a large amount of recirculation through the rotor disk, as the ground effect comes into play. I would think that could exceed the steady-state hover outflow velocities that are developed from the added inflows from recirculation. The hover case could achieve the same sort of outflows as the deceleration case if there are features that result in increased recirculation. For picking up into the hover, the ground effect starts off as a benefit reducing the power required but dissipates the higher the hover becomes. The increase in power required to commence translation in zero wind is less than 5% last time I looked. Power required is a 5th order polynomial for the hover through translation to Vne, and I guess it is for the decelerating case as well, except that the power requirement in the deceleration has a larger spike from steady-state compared to entering ETL in acceleration. Just a musing, but it suggests that doing an evaluation of a rotor wake effect HLS THA for pedestrians, tarps, etc is probably more arbitrary than an exact science. A downwind, confined space with buildings modifying the outflow would probably give quite a lot of additional wash.
Still a sad event