No. Those who have Down's are mentally handicapped. He may not have had the ability to understand he was "different", and so, not letting him play just seemed mean.
Assuming this girl is as intelligent and "firey" as you have tried to imply, she has no such excuse. She knows she has limitations and is demanding that everyone just put the standards aside because she wants to do something.
There is no different from women demanding special "tests" so they can become firefighters. They don't care about anyone else. They only care about what THEY want.
We are talking about a teenage girl, here, who has faced, and overcome, incredible handicaps and obstacles from birth. That's remarkable, and to be commended. She's totally unafraid to put herself out there, where many would, and do ridicule, or worse, pity her. It doesn't phase her. For whatever her reason, she relates to cheerleading, and would like to be a part of it..It excites her, motivates her..it's her hot button, whatever..
It would have been so easy for the coach, and the other adults in the school, to embrace this, and encourage the girls on the team to embrace her in a like vein..that's what I meant by a teachable moment...It could, and should, have been handled with grace, and love..instead, it's now a mess..