While it's not a case of out-and-out incompetent installation (unlike the
original "vent into attic" setup), it wouldn't be my first choice. Some
folks prefer to minimize the number of holes in their roof, 'tho.

Personally, I'd probably just pull down the fan unit that is getting spun by the
air current, and put a flapper inline with the outlet (many already have such a
beast...sounds like either this one doesn't, or it's not in the best of shape).