DID is a pretty serious diagnosis, so my advice would be to tread lightly there (not that I am implying you are diagnosing her, I realize you are not). But acting erratically and seeming to have personality shifts is something that is not limited to PTSD or DID - it's also a symptom of Borderline Personality Disorder and Bipolar Disorder. And people can go through that while grieving as well. So, it really could be a number of things; I would be skeptical of saying she actually has several different distinct alternate personalities, in the medical sense of the term, though she probably is experiencing some degree of dissociation.