Answer: 17 weeks low AFI could be a serious problem. Most often it's a problem with the baby, because amniotic fluid is mostly baby pee, but it could also be a placental problem. Does she have high blood pressure or a clotting disorder? Both of those can cause the placenta to function poorly and therefore the AFI to be low.
AFI is difficult to measure, so you should make sure that the same person is doing the measurement each time. However, an AFI of 2.5 is so low that I doubt they were off that much. Normal AFI is usually between 8 and 24 (I think), and that number starts to go down the further along you get. So at 37 weeks it is fairly normal to have an AFI of 6, and it's not a reason to induce or anything. They usually want to induce if the AFI gets below 5. Long term low fluid can cause death (cord compression), but usually causes birth defects, things like club foot because the baby can't grow the way they are supposed to.
bedrest and 2 gallons of water a day is a good treatment. They can hospitalize you and give you an amnio-infusion so the baby can stay in there as long as possible, but hopefully resting and staying ultra-hydrated will be a good enough treatment for you. It seems like most people who have this end up having a normal pregnancy just by watching their water intake and resting some. This is more true in the absence of confounding health problems.
I hope this was helpful to you