I do not feel remotely qualified to speak to your daughter's specific situation, so keep in mind all my comments are for a general case.
In general:
Imaginary characters can be a safe place to hash things out, especially if your real life interactions are few or generally failures. This can be true of 12 year olds as well as 6 year olds, and the ages don't raise any alarms for me.
I know people get worried about that b/c they are worried about schizophrenia. I am guessing that is your concern due to your comment about reality and fantasy. I know nothing about schizophrenia, but I would caution against assuming that just because your daughter prefers fantasy to reality that this means she can't tell the difference.
I am not remotely qualified to tease this out. I tend to think people worry more about that possibility than is warranted, but schizophrenia is a known co-morbid of ASD. In general, I think imaginary friends can be helpful and it is definitely a form of imaginative play, despite being solitary.
Immersive games can be difficult to extract oneself from, and that would be my main concern there. If she has the self-control to leave the game and do what she needs to do, then I don't see any harm. I am not a shrink, though, and I do not know if making what is in her head more concrete in a game is harmful in some way. I would think that it would not be because it is in her head anyway. I also have a strong pro-computer/pro-game bias you might want to be aware of when evaluating what I say.
Edited fr grammar and clarity.
Last edited by ASDMommyASDKid on 18 Sep 2014, 9:55 am, edited 1 time in total.