Welcome to the forum! The best thing you can do is be there for her. (I'm speaking as a sufferer who has suffered for 4 years myself. I'm just posting from the perspective of MY friends right now. They're my rock. :))
Some ways to help include: If she gets confused, doesn't know who you are, where she is, and so on, she is experiencing dissociation and she isn't joking if she tells you that she doesn't know where she is or who you are. Explain to her (Don't whisper it) that she is (however many) years old. She is in (put location here.) She was (just doing this activity with you) and she is safe. After this, she may continue to question, if she is still confused, just continue telling her this and repeating "You are (blank) years old, you are at (blank) and we are just about to (blank)." Don't get annoyed with her, and you MAY not be able to touch her. It's very very confusing when someone dissociates, and they may be freaking out- if they are, don't show that you are panicked too or it could make things worse.
FLASHBACKS: Ever watch your friend burst out crying and run out of the room? How about shaking and crying? They might be in a flashback. Best thing you can do is recognize when she's triggered. Even if you don't know her story, you might recognize what she does when she's triggered. If she just stares apparently at nothing ask her if she's okay, tell her that she's safe, and that everything is okay. ((That's one of my dead giveaways according to my friends.) Another dead giveaway is if she tenses up, 'makes herself smaller' and starts to bow her head or balls her fists up tight. Again, tell her she's safe, everything's okay, and if necessary, take her hands and lead her out of the room to a place where the two of you are alone. She needs to get to a "safe place" as she has just entered flashback mode.
If you are aware of these signs, you might be able stop a flashback in it's tracks by just telling her you're there, and she's safe. It can be very scary for you, but it's also very scary for her. Don't try to make her open up, but if she does, be willing to listen, and if you see her dissociating or flashing back, you may need to "play detective" if she isn't open (she may not be) and figure out what made her scared/sad. If you can piece it together, you can prepare her before anything happens as you clue into her "triggers", and if you know you're going to be facing them. That helps a lot.
Also, another thing concerning relationships, friendships, and so on.It's great to know that we have someone who wants to help and that we can lean on them, however they may become withdrawn or isolate. Allow them their space, it isn't your fault, but do make the effort to invite them to hang out.
For more information, go join the supporters section, and welcome to the forum! :)