I would just leave it at a polite, "she'll be back X day". I tend to look at both sides and when I join a group or am part of something ongoing, just suddenly not showing up would be unsettling to others whether they care about me or not, especially young adults, it would go beyond just curious or gossip I think; there'd be a hole there in there lives; I would have literally disappeared. I don't think it is so much about the mental illness (that they don't know about) but just the sudden gap that is disturbing to all of us on a basic level? If "it" can happen to someone else, maybe "it" will happen to me.

A general reassurance that she will be back, is not gone for good, should be sufficient. If they get nosy and ask additional questions, change the subject. Weather works well for that :-)