I think it is quite normal especially near the end of pregnancy as baby is engaging and probably resting on the back and pelvic more...not to mention becoming bigger and heavier too! It is not dangerous and just needs to be managed.
Also it is good to try to get baby in a good position for birthing. If baby is posterior it can cause very bad back pain so get her to try to lean forward whenever she can, or get on all fours. Sitting on an gym ball leaning forward helps, or she can sit on a chair back to front leaning forward onto the back rest. Lots of rocking either on a ball or leaning against the wall and rocking in a circular motion helps as well. Try not to slump back into a couch or be in positions where knees are higher than hips.
Other ways back pain can be managed: Massages (can be done by partner), swimming, heat packs and a pregnancy belt. I find the pregnancy belt helps a lot when my back gets really sore as I have back pain throughout the whole of pregnancy.
And rest lots!!
Hope this helps your cousin and all the best to her