I get this sometimes, used to get it a lot more. Things I was recommended: Specialized insoles with the metatarsal button, which seemed to help a bit, and wider shoes, which helped a LOT. For me, wider shoes was easy because my old shoes were women's, so I just switched to men's (which are wider for the same size), but it looks like your wife is already wearing men's shoes, so she'll need to find men's wide shoes if she wants to try that.
Once I'd switched to men's, the numbness only ever showed up on my left foot, which is the one I don't unclip at stops, so I started wiggling my toes every time I unclipped, to give both feet some extra blood. If she can't actually wiggle her toes around in her shoes, they're too tight.
It can go along with/also manifest as hotfoot, so recommendations for that also apply. Sometimes seat height or saddle position can cut off circulation a little bit on the back of the thigh, not enough to notice except in the toes.