Yeah, I think Davey's Mom is reacting as a lot of people would if they found out that their 12 year old had become a 60 year old over night.
In a real way, she must be grieving for the son she loved and raised for ten years. He doesn't exist anymore, he blinked out of existence without her even noticing, and now all she has is this man who's older than her, who looks just like him. And it wasn't an accident. This man became the person his is deliberately - and then put himself into her son, also deliberately.
No wonder she's resentful, and finding it difficult to just say 'I love you honey' and then going to work as if nothing happened, as if her son isn't, for all intents and purposes, dead.
Mother's react in very strange ways sometimes. When I was 14 I had a minor nervous breakdown and rather melodramatic suicide attempt (which involved listening to 'Angie Baby' on the CD player and gobbling paracetemol and sherry). My mother's reaction to this was to buy me a college textbook on the practice of cognitive therapy, and tell me to read it.
Sometimes I wonder I came out as sane as I did. (And don't nobody make the obvious comment! )