A lot will depend on where and when they are travelling.
If you break down in a remote area in Greece in summer then your needs are going to be very different than if you break down in a remote area in Scotland in the winter.
Also depends on the car, if they're in a small 2 seater sports car then space is a concern.
If on the other hand they're in a big 4x4 then they can throw pretty much anything including the kitchen sink.
I usually keep at least:
2 x 1.5l bottles of water
Hi-viz vest
Wet wipes
Some rags (usually micro fibre type)
a work glove
Warning triangle
Fairly comprehensive first aid kit containing items i'm happy working with.
Bottle of tyre repair foam
12v pump
Tool kit (socket wrench and bits) screwdriver, wire cutters etc etc
Torch
The usual zip ties and duct tape
In winter i'll throw in a thick 3 layer water proof high viz jacket and a wool blanket.
In summer i usually throw a couple more bottles of water in and a bottle of sunblock.
Slow down lads for crying out loud...while the original question sounds more like a request for help with a vehicle / bug out bag, we're talking about a car in Europe, not the end of the world; they ain't going down the trans Siberian highway in mid winter, they're not in Transylvania with the wicked Count waiting at the end of the highway devilish grin on his blood red lips as the sun goes down lol
Spandit in post #2 got it just about right, add some cash and maybe an anorak each and its job done.
Not sure how much travelling you've done mate, but i can tell you from first hand experience there are hundreds of square miles of areas where there is no mobile phone service around Europe.
When we stayed in Pitlochry the other year we had a 10 mile drive before we got enough bars on our phones to make a call.
We've had vast areas in Italy, Germany, France and Greece where we went for miles without a signal.
Head off on a quiet B road and you could be waiting at least a night before anyone passes you.
In my experience as a bloke that usually drives/rides around 4000 miles a year up/down and across Europe (UK and back) you really need to be equipped so you can survive 24 hours.
If you have enough room, then ideally enough stuff to survive comfortably for those 24 hours.
Sure you might not actually need everything, but i manage to fit all those bits in my little MX5 without any worries so it's hardly a hardship to throw a few things in "just in case".
We were stranded on Lesvos the other year when our hire car broke down.
It was 8 hours before we got back to the house.
4 hours were spent out in the midday sun with a car with no running motor (so no AC) with temps around 45c in the shade.
If i hadn't had thrown a pack of 6 x 1.5 litre bottles of water in the car we'd had been in serious trouble.
This in a island that's only just over 25 miles wide, with mobile phone service, in a country were we spoke the native language, AND had local breakdown coverage.
)
