By six weeks, bf is typically pretty well established. BF should not hurt. Some women do feel uncomfortable, especially when let down occurs, because the ducts are being stretched.
Can she describe the soreness?
Does it go away after let down? After a feeding?
Is it sharp?
Does the baby have any signs of thrush- diaper rash, white spots in mouth? Was mom on any antibiotics during delivery?
Nipple soreness can also be an indicator of a problem such as a plugged duct or thrush, so this may warrant some further investigation.