Considering Episode 5 is entitled "Ain't no Mountain High enough," I'd guess that Mountain High ski area is the one featured. Most things check out (trees, countryside, even the shed by the top of the quad chair), however the centered-bar on the quad doesn't seem to check out (via faint outlines of the ring-style support poles seen on satellite imagery). That feature (along with the retro connection to the cables) seems more characteristic of the lifts at either Mt. Waterman and/or Alta Sierra.... However the latter to ares only have double chairs. I believe Baldy (according to their trail map) only has double chairs.
At the top of the "East peak" at Mountain High is a Chair 8, a beginner's quad chair.
Then again, it is possible the ski resort did not want to be identified, so perhaps some disguising such as putting old quad chairs on the cable and/or CGI-ing in different chairs was done. It is clear that the location was not meant to be identified (at least from the footage linked).
If the description holds true, where there is "thin air" at "8000 feet," then Alta Sierra and Waterman hold truest to that, followed by the East peak at Mountain High (the West peak there rings in at exactly 8000 feet, however no fixed-grip quads service that area).
Of course, that's all ridiculous and it is probably Mountain High or some other resort I am not thinking of.