I had Oligohydramnios with my daughter. It's unexplained, but it was linked to her birth defect (alot of bubs with this defect have low fluid)

It caused severe IUGR and DD was delivered at 34 weeks.

Does she have an ultrasound report? On the report (or she could ask her OB), see what it says. It should have a measurement on how much fluid is there, deepest pool, etc. And I'll conpare it to mine.

I was drinking 3ltrs a day and that helped my fluid from reducing any more.