What have you done in a similar situation?
Nothing!
I learned many years ago that it wasn't my duty to tell adults how not to be stupid.
In very general terms, if someone hasn't learned what is or isn't a dangerous or life threatening action by the age of 25 or so then they're unlikely to learn from anything that I'm about to tell them. *
Similar for many other aspects of life. If you see someone driving like a F
kwit, do you pull up at the next traffic lights and inform them of the dangers of their actions? And if so, is your advice taken positively?
To play Devils Advocate:
what appears to be a dangerous action from my POV may not be dangerous at all, due to any number of things of which I may not be aware.
Late last year we were approaching Devonport at around last light. A jabiru heard us on CTAF and said that he was about to line up on runway 32 and that he'd wait for us to land before he departed. (All nice and good so far). During the landing roll on runway 24, we noticed that we couldn't see him on 32. He then departed and we could barely see him as it was dark by now. This bloke took off from an unlit runway and had no lights on his Jabiru. The Captain got on the radio and asked him wtf? His reply was "No problems, I'm only going around 5 k's."
From my Point of view, here is a guy risking his life flying an unlit aircraft. Taking off from an unlit runway, flying to an unlit airfield after last light when he could just have driven his car there in around 15 minutes. Sheer and utter stupidity! If he can't appreciate the danger that should be obvious to anyone, then nothing I'm about to say will make any difference.
But... he may have had NVG's, had SpecOps training (any military Porter drivers would know what I'm talking about), may have had lighting at his airfield, may have not put his lights on for some reason, etc.
DIVOSH!
*(I don't always follow my own advice
)