It may be simple. I sometimes get the "feet on fire" feeling. It always starts on the left foot first. I unclick with the right foot and at lights I prop myself up with the right foot and leave the left foot clicked in. So the left foot never gets to flex and get the juices flowing. I have found just walking around a little makes the feeling go away. I generally comes anywhere from after 30 miles to never.
I have found from experience that when my pedalling is sloppy, it causes the foot pain. If I pedal in "circles", I am usually fine. By that I mean, there are times when I don't really pull up on the up stroke but seem to slightly push the foot up with the power stroke of the other foot. That means the foot always has pressure on it and I think that constant pressure, over time, causes the foot on fire feeling. When I pull up, I unweight the foot and that allows better circulation. Maybe you wife will find that she is having the same problem and maybe not.