Rotor downwash is greater than that from an aeroplane of equal weight and so light aircraft must treat all helicopters with a great deal of caution. The vortices will move and behave in a similar way, just with greater strength. As with an aeroplane, the vortices are stronger at lower airspeeds and therefore the worst case likely to be encountered is from a helicopter in the hover. Even a helo with rotors running on the gound may still be generating some lift.
In my previous life, flying helicopters even larger than those that I believe TMM encounters, we were acutely aware of the damage we could do to light aircraft. Close to the aircraft, the wind generated by the downwash was in the region of 100kts - more than double the typical stall speed of a light aircraft. There is a video knocking about of a light aircraft taxying too close to a hovering helicopter and ending up on its back. Give 'em a wide berth.