Can rotorcraft pick up a dangerous electric charge?
In the same way than an aircraft will build static charge on the wings from friction with the air, the same thing can happen with helicopters from fast moving blades.
From a 1962 US Army document:

Any aircraft flying in the atmosphere can be considered as a conductive body located in a highly electrically insulated environment. In such a body, electrostatic charge is generated by three principal mechanisms:

Charge transfer created by a triboelectric effect between the aircraft and the atmospheric particles, such as dust, water, water vapor, snow, etc.
Unbalance of the ionic content of the exhaust gases produced by the aircraft engine(s).
Induction charge due to high electrostatic
field gradients which are found occasionally in the aircraft flight path.

Source

Here's a more detailed photo of the US army discharging a helicopter while wearing insulating gloves:

 SGT Nathan Rodeheaven uses a static probe to discharge static electricity from a UH-60 Black Hawk before his teammate, PV2 Danny Browning, hooks up the cargo to the helicopter. The safety measure prevents the hook-up man from being electrocuted in the event there is accumulated static electricity. Photo: Kristin Molinaro