Research, research, and more research.

It may be beneficial if you have a therapist in your life right now to ask your therapist - i bet they could provide lots of much-needed information.

IME with down syndrome children or adults, the most important thing to remember is much the same for children who are autistic - their brains do not process social things the same as we do. For the most part downs syndrome people seem to be EXTREMELY sensitive and emotional beings - they feel things intensely...and they don't always have the ability to process that in the correct way.

Keep in mind like nucking said that they lack motor skills- so anything valuable should probably be kept out of the way - just something to keep in mind, its noones fault that your camera happened to be there, but for future try and keep those things far away.

You may want to try agreeing to do some activity you know she likes - if she enjoys coloring, spend some time just hanging and coloring. Positive reinforcement works MUCH better with DS people - rather than getting angry, in fact, getting angry although it must be shown so they know you are a real person, is often taken much more harshly than it is intended - why she ran away because she probably didnt know how to process.

The most important thing to try and be is compassionate. Imagine if YOU yourself were that person - and you couldnt figure out why your friend was so angry. Imagine you didnt have the social skills to figure it out. It would probably be pretty embarassing and shameful. Also other people in her life may be treating her poorly and thats a shame too.