This is exactly the sort of nightmare scenario that (in the US, anyway) a durable power of attorney document is supposed to help with!
have a trust? LLC? non-profit? corporation? anything? whoosh, right over their heads
now? the exchanges all have this, pretty easy to onboard a whole hedge fund onto them now. but the crypto tax services, single-user personal account incapable of understanding other tax circumstances. there are ways (like exporting data to more robust software), just wish it was easier already.
but don't get me started on the on-chain stuff...