Serpentine belt derailed and big fluid leak

Hello:

I'm trying to diagnose a problem with my son's vehicle. It's a Jeep Grand Cherokee, which I'm unfamiliar with. And my credentials are weak: I'm just a weekend shade-tree mechanic.

Anyway, while he was driving he suddenly lost the power assist from his steering. He continued home, looked beneath the vehicle, and noticed a lot of fluid leaking from the steering gearbox area. He thinks about two quarts leaked just while he was watching. It was nighttime so he didn't do much more investigating.

Today I investigated. The serpentine belt is off its track. Not broken, just derailed and dangling; not riding on any of the cogs or idlers. There's a little bit of power steering fluid (I think) on a few inches of it. So somehow it derailed. Nothing is leaking at the moment, but I can see some power steering fluid wetting the area below the steering gearbox. Thats probably the source of the leak my son saw. However, the power steering fluid in the reservoir is at the "Full" mark on the dipstick ! The *only other* signs of leakage are near the radiator overflow port, where it looks like a small amount of coolant has leaked from the overflow hose.

So to me it doesn't add up to a clear root cause. Why did the serpentine belt derail? Did it slip off because power steering fluid leaked onto it? Why is the power steering reservoir full after my son watched "a couple quarts" of fluid pour onto the driveway?

Any thoughts or advice; tips for additional troubleshooting?

Thank you,
Jeff