Oh my!
My friend had a baby that cried all the time.
She took the baby to the Pediatrician.
The Doctor looked over the baby, VERY well and VERY closely from head to toe.
It was found, that a tiny blond hair, barely visible to the naked eye, was tangled and wrapped around her baby's toe... tightly, and to the point where it was now cutting into her baby's skin. The Doctor removed the hair, which was not easy because the hair was so tightly tangled on her baby's toe.
Once removed, her baby was fine.
Then, my other friend, her baby cried all the time too. By this time it was 5 months that had passed. I told her to make sure baby is latching on properly and that she is producing enough milk and to feed on demand.
Finally, she saw a Lactation Consultant. Her baby was not getting enough intake due to latch problems and she was not producing much milk. So the baby was basically ALWAYS hungry. 24/7.
Her milk would not increase etc. so then she had to supplement with Formula. BUT, since then, her baby was happy. Not crying.
Now, hopefully, that baby you watch, her parents know that too and have consulted with their Pediatrician?????
Now, since you said the baby is breastfed only. How is she feeding when she is with you or her Dad????
Does she take a bottle?
Or is she just not feeding at all?
If not, then she is probably very hungry.
Babies, need to be fed/nursed/given their bottle, on-demand. Not by a schedule.
Or have the parents tried Infant Gas drops such as Mylicon?
It is about time, they ask their Doctor if they have not done so.