The park scene at the end supports the "she hallucinated everything"
Remember how she goes all over howling and yelling to the top of her lungs for Sam, right in the middle of a crowd of parents and kids, yet NOBODY seems bothered or surprised in the least?
How many of you wouldn't at least turn around to see at a woman acting all crazy like that?
Yet nobody seems surprised, least of all her son, whom is acting like everything's cool and normal, which would make sense if indeed people are familiar with her illness already (local park, local crowd, people already know her)
And Ash at the end, he does indeed acts like he's already familiar with her and looks at her with a sad look after she introduces herself and he has to specify "huh yeah, we've already met".
But I agree there's plot holes to her hallucinating everything:
a) Is the husband real? He has to be for otherwise
- how would she afford such a townhouse? Clearly she can't work nor is a trust fund baby.
- Sam would never be left in her care if she was indeed living alone. It's just not possible.
b) What about the shrink addressing her son's death if she never lost him?
- Most people whom support the hallucination theory point to the coffee scene where she starts hallucinating. Yet the shrink is discussing her coping with the son's loss before that.
c) If she's indeed that far gone, how come there's no mention of medication at all?