Original order was in New York child has down syndrome 21 yrs old but now she and custodial parent live in puerto rico

So what is your question?

So what is your question?
Can her father stop the child support now that she just turned 21and is still in school ?

Depends on what your original support order stated. NY law orders support until age 21. But if the order included a provision to continue while in school then it should continue.