There seems to be a lot of barely informed twaddle on this thread, coupled with what can only be described as 'idiocy'.
I am forces, I work with Cadets (ACF, CCF (Army)) in my spare time, and also a youth club. When I was younger I was in the Boys' Brigade. The BB Uniform for boys 11 - 18 was a blue shirt.
Your DD1's friend is obviously on some sort of formal parade. It may be at odds with modern life, but the SCC have a dress code, this is enforced with a little more rigor than school uniform.
Cadets of all hues are PROUD to wear their uniform.
So they have been told that they will be on parade, for a period of time, in only the prescribed form of dress.
SCC WILL have warm kit. They are issued a supply of Gore Tex Navy pattern water proofs of the SAME standard the RN and RNR are issued for use on Ocean going ships. This is expensive, thus such items are usually held at the Training Ship the SCC is based at (a TS is usually a building by the way) and issued and returned as needed.
Before this parade, it is possible the £100 odd a shot coats will be issued.
Failing that, your DD1's friend has been warned that coats, etc. will not be worn, and they'll be outside for a few hours.
To that end they would be well advised to dress in layers UNDER the uniform. And no doubt they HAVE been advised to do so.
I wore a SHIRT, as my uniform with the Boys' Brigade. Under which I often wore sweatshirts and thermals as needed. I am still alive and healthy.
SCC wear a nice Wooly Pully in cold conditions, IIRC. Under which a sweatshirt or thermals will keep them nice and warm.
Their hands may be cold, but if their core temperature is maintained, no one will be harmed.
This is all good character building stuff. If they get cold on this parade, they will not on the second. I doubt the Adults will stand by if a Cadet truly needs to be fallen out and warmed up.
This standing about (as no doubt some of the above cretins see it) will teach a valuable life skill to the Cadets, so that when they are in more adventurous environments such as canoeing, sailing or mountaineering they will be able to dress THEMSELVES appropriately.
All these things I did, Duke Of Edinburghs, canoeing, sailing all before I was 18. Letting our kids fall (a little) in a safe environments allows them to develop in to the Adults we want them to be.
Furthermore, don't assume that just because the adults are uniformed they don't care about the kids.
As an aside, Hulababy, I very much doubt you have a clue what heat exhaustion really is. I can assure you the kids fainting on parade were no where near heat exhaustion.
As for risk assessments, NO Cadet activity will take place without one, so don't imagine demanding 'sight' of risk assessments will suddenly cause panic.
Pointything, it is a shame that DD1 has not gone in to the SCC or similar. The experience, responsibility and fun that is available in CONTROLLED and SAFE environments really develop kids.
Google Ten Tors or Duke of Edinburgh's Gold Expedition to see what some of the Young People are capable of 'on their own' with proper training and development.