Can anyone offer me some advice?
It's pretty common what's happening to her. That might be a good start... "mom, fixation on numbers is a common mental hangup, relax, you can let it go"
Personally I'm haunted by 324, which was the favourite number of a teacher I hated. Also 23, for obvious reasons.
The trick is to understand that this is mostly just your mind playing tricks on you, a bit like a "self fulfilling prophecy".
Too bad I forgot the correct term for the mental illness that describes this phenomenon, but reading about it alone should help a bit with relieving her. The other problem here, obviously, is the trauma from what happened on that day. So even if she manages to get rid of the number thing there's still that to overcome.
But to me it seems (I dont know her) this is a pretty simple problem that can be solved with a bit of talking. Of course, every knot in your mind is tied to another knot that happened earlier in your life, so if she sees a therapist about it, chances are, they'll try to milk her for what it's worth.
On the other hand, if she had worse traumata as a younger person, those might as well be the reason why she even was able to develop her current problems...
edit: If she was my mother, I'd ask her about how she feels about that day and whether or not she stills fears "terrorist attacks". If I'd notice anything that might be the trigger to get the "emotional waterfall" going, it try to tickle that thing a bit, and just listen (or even just pretend to listen, whatever, just let her do the talking).