My advice to them would be the same: don't try to rely on, or memorize, a hierarchy. Just know that all of these phrases are relative, and it depends on the speaker and the conversation. But I know people prefer to have concrete answers, and "its all relative" sounds like mealy-mouthed advice. Maybe I would turn the question around to them and try to have them rank Japanese terms:
かなり高い
やや高い
けっこう高い
なかなか高い etc...
They can't really be ranked. Of course, a super opinionated person might claim that けっこう高い is always and irrefutably higher than かなり高い, but these rigid rankings fall apart pretty quickly. I just did a quick AI google search for the terms "quite" and "rather", and it spit back this gem
"Quite" and "rather" are both adverbs that indicate degree or emphasis. "Quite" can mean "a little, moderately but not very" or "very, totally or completely".
Accurate, but very frustrating for the learner of English. And also the frequency of usage would be different between the UK and the US, which adds another complexity. I think a learner of English needn't be bothered by learning all of the subtleties of usage of the word "quite". It's sufficient to know that it emphasizes the adjective that follows.