A good friend...

Could it be the A/C drain is plugged?

Some are just totally rude when it comes to personal property.

- Greg

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I'm with Greg - sounds like a plugged A/C evaporator drain - a very common problem. Not positive about the Prius setup but in most designs even a leaky heater core won't overflow onto the floor if the drain is clear. The last time I had a bad heater core it thoroughly fogged the entire windshield. It seems warm coolant produces a lot of steam

The sweet odor indicates a leaking heater core. A good friend would urge the owner to get it repaired promptly.

The "musty" description doesn't fit, though. A leaky heater core produces a powerful smell similar to vanilla. My nose doesn't work well but I can sure smell that. The musty smell is more consistent with a plugged drain and mold growth. A check of the coolant level will quickly determine whether it is a heater core leak; if the passenger floor is wet with coolant the coolant level will have dropped dramatically. A paper towel used to blot the liquid from the carpet should also show the red dye used in the coolant.

In either case I agree it should be fixed promptly. Mold is very bad stuff and can be hard to control, and loss of coolant can cause serious engine damage all too soon.

Mike