Dealing with the mentally handicapped

This is the first week where Katie and I have had to deal with a truly mentally handicapped kid. At the audition we weren't entirely sure if there was something wrong with her because she seemed pleasant enough and didn't have any physical handicap or anything so Katie and I gave her a pretty big role because she is 18 (the oldest by far at the audition). Well tomorrow is the big show and although there has been marked improvement throughout the week I am a bit nervous because she had an outburst yesterday during lunch that took a lot of reasoning on our part to work through...She was wearing a shirt today that said something about Down-Syndrome even though she doesn't look like a downs child which threw me off a bit. I was trying to talk to her mother after, to get ANYTHING out of her as far as what we might be dealing with but she just smiled and kept saying, "This is so good for her!"
At any rate all I can do is just be super prepared for tomorrow and put it all in God's hands...