[gremlin]

Originally Posted by Alpha

Nah, I understood. Same sort of thing she did to restrain the Didact. Hate that she's gone....

I don't believe that she died. You don't see her die, she simply disappears and she just can't go with John this time.
I think that she will reappear, there are rumors that she will return as a human. Due to the fact that she is technically a clone of Dr. Halsey. Apparently they will find Forerunner technology that can bring her back as such.