In reply to cjbobo88:
>"try and get it through to them that this stuff is important?"
If your friend wears a pair of Walsh running shoes, Ron Hill shorts and calls himself a fell runner I think he'll get away with it.
It's difficult to appreciate how harsh the summer mountains can be when most times a walk up a popular UK mountain with a good trail, in fine weather, really doesn't push the envelope of human endurance.
I'd just roll with it. Perhaps after the first summit when a biting wind has chilled his body through a sweat drenched cotton T-shirt, he might learn that some additional clothes might be a plan for the next one. But let's face it, your friends will be unlikely to come to any serious grief if there is at least one person in the party half prepared like yourself.
There is a balance to be struck. Being burdened with the weight of over preparedness really can suck away the enjoyment and feeling of freedom in the hills.
Go on, take a risk. If the forecast is good, leave the gaiters and waterproof trousers behind.