I got the impression that Helen knew that it was really Adam at the end and not Anthony based on her asking him how school was and later telling him she wanted him to stay when he was apologizing. Also she must've noticed he didn't have a ring, just as Mary was tipped off to Anthony. Is there any confirmation of this anywhere either in the movie or other commentary?
In the scene where she meets "Adam" at school. She's shocked. "Adam" asks her about her pregnancy.
She talks to Adam and is heartbroken. Observe here that Helen is not surprised, she’s upset. She’s seeing Anthony who appears to be acting and talking like an Adam. She beings to understand that this version of him seems to be an alternate personality kept away from home. He’s compassionate and converses sweetly with Helen. He cares for her pregnancy and asks how far along she is.
Towards the end of the film, Helen is in bed.
Adam is being nice to Helen. She probably feels the way she once felt when she first met Anthony, before his cheating started. She suspects that the person who’s in her bedroom is actually the history teacher because of his mannerisms and he brings up that she is 6 months of pregnancy. She asks him to come to bed. He does. She looks at him longingly. Then she asks “Did you have a good day at school?”. This confirms that Helen knows which personality she’s with.
Helen wants "Adam" to stay. He's crying and he apologizes Helen holds his hands. They show a ring on her finger and not on his. But Helen already understands that her husband has two personalities... she wants to be with the guy who she thinks will love her not just the one she's married to. The ring means nothing to her, it's the relationship that she's interested in.