At first I thought she was the woman who was attacked by Abby and then turned into a vampire in the hospital. It was as if she just disappeared, never to be seen again.
Eat some makeup because you are really ugly on the inside - Olivia Blois Sharpe
"At first I thought she was the woman who was attacked by Abby and then turned into a vampire in the hospital. It was as if she just disappeared, never to be seen again." - kelliemurray
Not really sure what you mean by "...she just disappeared." Owen's mother didn't leave the apartment except that one time to go to the school with Owen, after he had hit Kenny with the stick.
If the film didn't venture back into the apartment then you weren't going to see Owen's mother.
Her presence is vague, deliberately so, to show that she is of little help to Owen when he needs her the most.