Over the years (okay, I'm sounding old, it's not that many years - but this fact increases the pertinence of my point) I've had some notable experiences in which the people I went mountaineering with weren't prepared, and it made for a ruined trip. Luckily nothing really terrible has happened, but looking back there was some serious potential for hospitalization or death.
One experience I still shake my head about was taking my friend, whom we will call... Jack, to climb Gannett Peak in the Wind Rivers of Wyoming. Jack is quite outdoorsy, he and his wife have enjoyed many backpacking experiences together, and when preparing for this trip he read just about everything he could find about such an attempt, not to mention he was in the Marines. I figured "Hey, no need to give this guy a packing list nor to specify what type of gear is okay and what isn't." So off we went, almost 20 miles into the backcountry, ascending a snow-covered Dinwoody (Bonney) Pass, preparing to cross two glaciers and traverse the ice cap to the peak, when he tells me how cold his feet are getting, and that his boots keep twisting sideways in his crampons. Apparently he had brought light, flexible, non-water resistant day-hiking boots to go mountaineering. Well, he wasn't feeling up to the task once we were up to the pass, so we start to head back to camp, which was lucky because it began raining. Then it was pouring. Then sleeting. And snowing. And hailing. And our tents were covered in half an inch of ice. And this poor guy had completely soaked feet. Interestingly enough, two weeks later Jack went on a university-sponsored rock climbing trip to the Winds and the 'instructor' had no idea that sort of weather was common in the summer. sigh.
On another occasion, in the Wind Rivers again, I had two other friends with me, neither of whom brought waterproof jackets. One was wearing a cotton hoodie, and the other a non-water resistant wind breaker (with his JEANS). Another rain/slush/snow storm ensued (actually, the only time I HAVEN'T seen such a storm in the Winds was the last time I went, which was mid-August). They were miserable. Luckily it cleared up and we found some dry wood for a fire so they could not only dry their clothes, but their sleeping bags as well.
Neither of these experiences ended up being really awful, but someone could have easily ended up with hypothermia or a broken ankle or sliding down a glacier into the terminus - a couple dozen miles from the nearest road. Too often we might assume climbing buddies know what's going on and we don't want to make them feel like we think they're inexperienced - but their life and health (and on a more minor note, the objective of your trip) is more important than hurt pride.
Mountains are extreme places, and most people don't realize how extreme. Sometimes people think I'm joking when I mention hurricane-speed winds combined with rainy snow. They don't realize there are places only a few hours from their homes in which it literally freezes over every night of the year.They have a hard time imagining a nearby location where ice chunks are blown up hills in July. It's not necessarily their fault (or maybe it is - it's often difficult to determine assignment of blame in the case of ignorance), but as one with a knowledge of how things are, it is YOUR responsibility to make sure your party members know what they're up against.
Next time you head out on a trip, make sure everyone knows what they should bring and the conditions they should be prepared for. If you don't, someone could die, and and you could have prevented it. Friends don't let friends be stupid. Climb safe!