Using “one” with nouns whose countability is ambiguous

An ESL student produced the following sentence:

“The Western diet seems to be more unhealthy than the Japanese one.”

The use of “one” immediately struck me as awkward, but not necessarily incorrect, as diet can be both countable and uncountable depending on the situation.

I corrected the sentence to, “The Western diet seems to be more unhealthy than that of Japan,” to avoid any confusion, but I honestly couldn’t decide if her version was actually grammatically incorrect or merely grossly awkward.

When using a strictly uncountable noun to make a similar sentence, such as, “The Western furniture was less stylish than the Japanese one,” it’s obvious that it’s incorrect.

However, when switching back to a noun that is both C and U, such as in, “The Western environment is less polluted than the Japanese one,” it becomes ambiguous again (to me anyway).

I realize that the best solution is to just fix the sentence as I did in the first example, but again, what I want to find an answer to is whether what she initially wrote is grammatically correct or not.

tl;dr Your student is not wrong, and the correction is unnecessary.

The countability isn't exactly an issue, in this case the one is effectively acting as a pronoun (to avoid repeating the word diet).

Your correction, sadly, is wrong because the diet is of the Japanese, not Japan. You should finish it … that of the Japanese to be correct (and I fear it sounds more awkward - and somewhat academic in style - to boot).

To suggest that “her version” was “grossly” awkward I find offensive - it is no such thing.

Western furniture might almost be considered countable in a sense, but yes, that sentence is clearly wrong.

The environment is either a single thing (global), or a local condition, and as such every city, town, village, square foot, person and animal in the West has its own (local) environment - The environment is less polluted in the West than in Japan [but I'm not convinced of this, having seen some of both].

The best solution would be to admit you were wrong and apologise to her.