Originally Posted by willcycle
Any ideas about what may be causing this?
As already mentioned, proper shoe fit is the first thing to check. Her toes should have wiggle room and not be wedged into the toe box of the shoe. The rest of the shoe should have a firm, but comfortable fit.
Riding technique can also be a factor in that "pedal mashers" can often times have more problems with "hot spots" or foot numbness than spinners who pedal in circles and "lift" on the upstroke instead of having constrant pressure on the ball of the foot.
Just some things to check on and to have her remain concious about while riding. Also, at the onset of numbness, she should ask you to coast so that she can loosen the straps of her shoe. If later on the shoe feels loose, just have her adjust it again. It's not at all uncommon for feet to swell during hot weather riding and/or for shoes to stretch as they begin to absorb moisture so make sure she knows that it's not at all unusual to adjust the straps on her shoes during the course of a normal ride.