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Bath fan vented through furnace flue?

After showering during a visit to my in-laws, I felt water dripping from the fan in their newly remodeled bathroom. The remodel included adding a fan in a bath that didn’t previously have one. I went outside, noticed no conventional vent termination in the roof, so I poked my head into the attic, and saw it was terminated into an existing flue (assuming it’s the furnace flue, 6” rigid pipe).  Given the dripping, and the fact they didn’t properly vent through the roof, I wouldn’t be surprised if they simply taped it to the flue without cutting into the existing pipe, but if they did cut into the pipe, a few questions:

1. Photo 2 shows pvc pipe that runs up through 6” flue, so is the pvc the furnace exhaust, and is the 6” pipe operating as fresh air intake?

2. 1. I know this isn’t correct, but Is this a serious safety hazard, or just a shoddy shortcut that may introduce extra moisture into the furnace, but is otherwise harmless?

3. Anything else I should look for or know before I call the GC and read him the riot act?

note, the house is in California, and the bath is only a few months old, so the furnace has only been run a few times since this was installed. 

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