Had a fellow rider blow a few turns like that down in Mexico on the Espinazo del Diablo. When he failed to understand my concern I explained that I was at risk at least as much as he was since an oncoming driver will be trying to miss him while heading directly into me following behind. Been there, done that.
If you find yourself riding with someone who doesn't practice good "lane discipline", as SOG put it, you need to solve for that straight away. Life is short enough as it is.
Hope this rider gets the chance to learn and get back on a bike.
"You don't take photos for yourself, you take them for the old man you will become - if you are lucky." - Falang
"As long as there's a horizon and I can see it, then I want to know what's there, mentally, physically and visually" - rtwpaul