How do you help a Downs Syndrome girl?

I have known this girl for nearly three years now and she's improved a lot from what I can see, but yesterday she was blaming my other friends for stealing a ring from her boyfriend's finger. He was trying to tell her it had fallen off and he'd lost it, but she kept saying they'd taken it, so in the end he gave up and agreed with her. Is this all you can do?

And what about when she gets tired and starts crying randomly? A while ago, she had jet lag and knocked her coke over, nearly destroying my camera. She ran out of the food court crying. I tried going after her but she told me to leave her alone, so I went back to help clean the table up. She did come back after a while, but was I right to let her go? Would she have back quicker if I'd followed her?

What should I do if this happens again?