He called out his plumber that had installed the backup system. The plumber mentioned something about debris being caught in a valve, which caused the city water-fed backup system to be discharged into the sump pit! Since the float on the main pump was apparently stuck, and the backup sump was discharging city water into the basement, that explains how the basement got so flooded. Unfortunately, my neighbor was at work while the plumber was at his house and he couldn't personally verify what the plumber was telling him, so we'll have to assume the plumber was correct and not just trying to cover his a** for an improper installation.
I'm assuming the whole event got triggered due to the normal drain water filling his sump pit, and when the main pump didn't pump out the water (due to the stuck float), the backup system kicked in. But when it kicked in, it apparently sucked up this debris which not only screwed up the backup system's ability to pump the water out, but also began discharging city water into the pit. I looked at his system this evening. Each pump had a check valve installed between the pump and the common pipe used to deliver the pumped water outside. The valve that the plumber claimed was full of debris was down at the bottom of the backup pump's pipe. I couldn't see very well down there, and given my limited knowledge of these systems, I didn't really know what to look for. But I can't help wondering how this "valve" which is supposed to suck water up out of the pit, can fail in such a way that it can actually discharge water into the pit. This seems like a major flaw with these types of systems if it's true. Is it true that they can fail this way? If so, how does it happen? Is there a diagram of these "valves" that show how they work?
Thanks in advance for any information.