hi cathy,
i realise your post is a little while ago but thought you may still appreciate some input...
might not be a case of one or the other - excessive traction on the peroneus can cause the cuboid to sublux.
have you had a feel of the foot? i've only had a couple of patients with this but in each one you could palpate that position of the cuboid and feel a difference left to right. often there is a depression on the dorsal aspect. or the lateral aspect of the cuboid may be more prominent.
the good news is if you diagnose it as a subluxed cuboid - the treatment can be a magic quick fix. you can manip it to reverse the subluxation and all of my patients felt an immediate improvement. you can then look at taping to maintain the position while you sort out the peroneus if that is the issue.
if you are not confident with a manip or your palpation i would try doing a dorsal glide of the cuboid and see if that changes her symptoms.
hope that helps, you may have cured her by now
cady