Hi--thought I could shed some light. My ds (now 9 mos.) was delivered at 37 weeks b/c of low fluid.
Low fluid is more serious the earlier it occurs in pregnancy. Very early on, it can be a sign of very serious things, like PROM, or Potter's syndrome (baby has no functioning kidneys, therefore does not produce urine, therefore fluid is low). Late in pregnancy, fluid naturally decreases, which may mean as little as the body is just preparing for birth. At any point it can indicate that mom is dehydrated or that there are stomach or bladder problems with the baby.
My low fluid was discovered at 30 weeks. I started to measure small, so I had an u/s where my fluid level was assessed at 2cm. I was sent for an IV and my fluid was then assessed at 6cm (still low). For the next 3 weeks I had 2x weekly NST and u/s. They assess fluid in each of the 4 quadrants of the uterus, and some drs will only give you "credit" for fluid found in pockets with no cord. So throughout this time I had some drs. who said I had 0 fluid,because there was cord in each pocket, and some were saying I had 7 or 8 centimeters. Sometimes I was told I should deliver immediately, sometimes I was told it would be better to hold off. Since u/s showed a normal baby, the main concern was cord accident, which was really terrifying to me. From 33 weeks on I had N/S tests every day, u/s every other day. My fluid levels were between 0 and 4. At 37 weeks I decided to deliver. Baby had stopped growing and I had previously delivered a 35-weeker and 36-weeker twins without any "prematurity" complications, so I felt in my heart that this one would do ok at 37 weeks. Also, the fear of a cord accident was getting to be too much for me, so I felt it was time.
Not sure about your friend's situation, but I think it usually comes down to a risk/benefit ratio. I didn't "have to" deliver my son at 37 weeks. He was doing fine on NST. But I was comfortable with the risks of delivering a 37 weeker, and not with the risk of a cord accident. However, when I was first presented the option of delivery at 30 weeks, I felt the risks of prematurity were far greater than the other risks associated with low fluid, so I chose to hold off.