labor is a lot like menstrual cramping. it can happen in the lower back, anywhere on the belly. it starts out not so bad and pretty infrequent, and when it's true labor, it will progress to the point where it's regular cramping that lasts for a few seconds to maybe 30 seconds and then goes away. When she begins to feel the cramping has some regularity to it, she should time her cramps (contractions). At this point, she needs to call her OB/GYN and let him or her know what's going on. When they are consistently 10 to 6 minutes apart and possibly but not necessarily quite painful, she needs to get herself to the hospital.
In terms of water breaking, as you've already read, it does not always happen, but if it does, they will make sure the baby is born within 24 hours or so, because w/o that water, the baby can't stay in the uterus safely for long. And yes, I think it will ruin the mattress. It's not like a gush of water usually, though - it's more like not realizing that you're peeing....
A due date is pretty arbitrary within two weeks plus or minus, by the way, so all of this could be happening tomorrow, or it could be December 1. You never know.
I have had two kids, just so you know. I had an epidural with the first, and no drugs at all with the second. I was scared to death with the first, and not scared at all with the second. I did not even experience any pain at all with the second until the last maybe half hour of labor. my waters never broke.
I strongly suggest you look up something called "hypno-birthing." it's basically a self-induced trance that works to totally remove the laboring mother from pain. I did not do it with my first child, and I think that the pain I felt made the labor three times longer and scarier and just plain awful. I was just so scared because of the pain. I was afraid of each next contraction. but with the second child, I removed the pain from the scenario (can't tell you how i did it - i was in a trance basically...) and it went so smoothly.
I learned the hypno-birthing stuff in the last weeks before the labor. I don't have the materials any more, but i bet you could google it and get it off the internet if you wanted to. There are probably also books out there on it - it was amazing, I'm telling you.
...but the best advice at all - make sure your health insurance is all in order and then...
SLEEP
best of luck!