Luke's mom is the spoonful of sugar in Luke's medicine. Not many people can put a positive spin on having to hide 24/7, but Luke's mom does. She's always at the ready to give Luke a "reassuring squeeze" (1.7) or give his hair a good rustling just like the itty bitty baby—we mean, adolescent—that he is. Yep, we do get the feeling that Luke is just a little too close to his mom.
That's why her new job might be the best thing that ever happens to him. At first, Luke is devastated when she gets a job to bring extra income to the family and no longer spends all day with him. Most noticeable is the absence of her making food which "sent good smells throughout the house" (7.10).
But in the grand scheme of things, her daily departures may have helped Luke flee the coop. See, it's really hard to sneak out of the house when someone is constantly there. So Luke's mom sticking around would definitely be a deterrent. In addition to that, Luke might not have been so utterly bored and lonely if his mom was there to keep him company thus removing the need to escape and investigate the mystery child he saw in the window.
In the end, though, Mrs. Garner is a positive force in Luke's life and probably the source of the only happy memories he'll have of his childhood. Before he leaves for good, she makes the most of every minute they have together; "filling him up with memories" (30.23). Aw.
What with the bread-baking and the home-making, Mrs. Garner comes across as an extra maternal type of woman, the kind of lady whose house would always be full of neighborhood kids. So, it's not exactly surprising that she decided to break the law and keep the child, when she unexpectedly finds herself pregnant. (The book is all vague, but the idea seems to be that women are sterilized after their second child, and if the sterilization doesn't work they're supposed to have abortions.)
Between aborting a wanted baby and breaking the law, we get her choice. But the way she frames it, that she would even let Mr. Garner consider other options, frames her as someone who's all about heart rather than head. Like Jen, she goes with her gut; she's not cautious and thoughtful and planful like Luke.
And in this world, that puts her at risk.