I am not the mother of a child with Down's but am a big sister to one. I recommend RESPITE! Offer to do fun activities with the child or have the child overnight for a weekend or whatnot. Give the parents a break! Other things that may work is helping transporting to activities if the child is in special olympics...and if not involved, perhaps find out about your local SO and see if they are interested in having her join. Perhaps a mentor may be nice as well. Those are the things that my family would have appreciated from people we trusted.
HTH
HTH