As I understand it, the "bombs" were made in the garage, whereas the mother lived in the residential portion of the home. Presumably the bomb maker didn't leave the evidence lying around when he wasn't working on it (say, when the mother wasn't home or when she was asleep). It wouldn't be the first time that criminal activity (drugs, pornography, and yes, bomb-making) went on at a residence without a mother having definite knowledge of it.
Does that mean the mother didn't know? Not at all. But in and of itself, the fact that she shared a residence isn't sufficient to conclude that she knew about it.
Why do I put "bombs" in quotes? One of the odd aspects of this case is that Farook supposedly spent months making pipe bombs with his neighbor (the one who purchased the two rifles). The neighbor says they did this "as a hobby". This is the same neighbor who says that several years ago he and Farook had been plotting a different attack, that was never acted upon; and also that there are numerous terrorist sleeper cells in Los Angeles.
These were black powder pipe bombs, which anyone can make. And apparently they did. For months. Using Farook's machine tools in his garage. So I would assume that Farook would know how to make a working pipe bomb.
Yet, none of the bombs known to police seem to have worked. During the police chase one of these pipe casings was tossed out. No explosion, and from later reporting, no explosives. It has also been reported that bombs were left at the center where the shootings took place. No explosions. It was also reported that bombs were left as booby traps at Farook's home. Then it was reported that no booby traps were left. Finally, it was reported that a dozen pipe bombs were found at the home.
Why did the neighbor purchase the rifles, but not the two handguns?
Why did Farook and Malik drive very slowly down residential streets during the police "chase" and why did they turn on their emergency blinkers as they did so?
Why did the FBI allow reporters access to what should have been a sealed crime scene just shortly after forensic examination, instead of keeping it sealed in case (as often happens) something new or overlooked or reconsidered later developed, or a lab test or evidence collection needed to be redone? Why did reporters need an open-house press party at all?
Why would Malik post a Facebook comment pledging allegiance to ISIS, at about the time of the shootings, after she was committed to overt acts of violence -- but do so under an alias? What was the alias, exactly what time was the post made, and how is it known that she posted it?
I'm not implying anything with these questions. These are simply a few of the questions I have, after watching news reports. Perhaps they can be answered here.