tl;dr if they’re not using Firefox you can send them a Clear-Site-Data: “cache” header from the same origin.
e.g. in PHP, send them a header
This can be on its own standalone URL, e.g. https://example.com/clear-cache.php – it clears cache for the origin, in this case example.com.
Firefox doesn’t support this header yet.
Open Dev tools, turn caching off, visit the cached redirect URL again, cache should be cleared.