I think your friend should stop being so "softly softly" with the child and realise that her approach isn't going to work with this one - she's his PARENT and she needs to step up and actually PARENT him before she does get reported for neglect or cruelty!
It's ridiculous to kowtow to the child's refusal to wear a coat on a cold wintry day - it's not good for him to get so cold!
I don't see any need to tell your friend that other mums are "gossiping" about her - but I do think it would be an idea to tell her that other parents are flagging concerns and that they might choose to report her over them because no one wants to see her DS suffering, however much it's his own choice.
I'm sure (well hoping, anyway) that if he told her he wanted to drink beer, she'd refuse to allow that. If he wanted to play with matches, she'd refuse to allow that. They're a bit more extreme, of course, because much more damaging - but the principle is the same: just because he wants/doesn't want, doesn't mean he gets his own way!
God, he's going to be a nightmare as he gets bigger if she doesn't nip this in the bud now.
If it helps, you could ask her if she read the Anne of Green Gables series when she was a child - especially Anne of Avonlea, when she was the teacher in the school. She believed in "winning her pupils' affections" and refusing to discipline them. Worked on most, but not on one child who thought all women teachers were too soft and no good. Different era of course, but one day he did something that made her angry, and she ended up smacking his hand with the ruler - and THEN he decided that she was a good teacher and behaved himself. So she had to go against her own principles to achieve something with this one boy.
I'm NOT advocating smacking - of course not! - just pointing out that not everyone responds to "carrot" treatment, some people need/prefer "stick" treatment to achieve results (donkey model of management); and that this needs to be recognised and behaviour modified appropriately.
Hope you get through to her!