Thank you for your query.
Mottling of the legs is definitely not a sign of labour.
In fact it is not normal.
SHe should consult her doctor, because purplish discolouration could indicate venous thrombosis
It could signify some rare skin disease, or purpura
due to deficiency of platelets.
In any condition, it is not a common or normal finding.
Besides, being 5 days overdue, she should definitely consult her OB GYN.