I just don't think hair would ever get that bad. When I had an issue with my DD's hair (which was entirely my own, and I totally recognised that), I usually managed to get them to let me brush it somehow. We bought a tangle teaser, we bought that leave-in conditioner, we made it a fun part of bathing with bubbles, offered to use my hair dryer on it.
It was never such a problem that their hair ever got as bad as your describing and to me, that is not UP or non-coercion, but neglectful parenting. It does not warrant 'rules' though, it warrants creative thinking, sharing best theories, working together to find solutions that please all parties.
Not coercing children is not rocket science, but does require a huge change in how you think about children and about your role as a parent. I don't think that the parent of the child you're referring to is doing that.
My children have had periods of having wild, unkempt hair, and they've never choked. If they had done, I don't think it would have taken much discussing for them to realise that it was vitally important that their hair was brushed. If they refused, then maybe I could just tie it back with a hair bobble. The one time we thought they could have caught nits (they hadn't) they trusted us when we said we needed to wash, condition and comb through their hair carefully.
The point is that they trust us because they know we don't ask them to do things that have no rational basis and we will always strive to find a solution that they are happy with as well as us. Children aren't born irrational, but they very quickly lose that rationality when they're not trusted.