People always underestimate helicopter downwash and often play token service to what constitutes a proper tie down when "mixed operations" are required.
I was once parked on the tarmac helipad at a certain grass airfield, for a refuel. A Piper Cherokee or similar was taxied up to a position less than ten metres away and was shut down. The instructor and student got out and began walking away without tying the aircraft down. I politely asked if they were going to leave the aircraft parked there for long because I needed to depart quite imminently and we would need to lift to the hover to leave the helipad. The instructor casually said they were finished for the day and continued to walk.
I pointed out that the helicopter weighed over five tons and produced a very strong downdraught which could cause damage to their aircraft, or even lift it. The instructor very reluctantly (and grumpily) stomped back and tied a coupe of 20 litre water containers under the wings, then stomped away again....
Thankfully, I was able to lift off to a very low hover to clear the the raised edge of the helipad without causing damage.