Mary,
It is the property manager’s responsibility to remit the taxes which is usually the client, a trusted relative/friend of the client or a third party property manager. It can be the accountant - you - but I wouldn’t recommend it due to being on the hook for tax payments if the client doesn’t make the required remittances. It is supposed to be the person who is actually collecting the rent and has a Canadian mailing address. Penalties/interest are the responsibility of the property manager (which why no tenant in his right mind would take this on) and also making sure the returns are filed on time. I’d file the NR6 now for 2021 and 2022. You should get a response in 6 weeks or so and then not have to remit the taxes for say Oct 2021 onwards. Make darned sure the S216’s are not late (June 30th - 6 months after year end) as if they are then the expenses are all disallowed and the tax is a flat 25% of the rent collected. NR6’s should typically by filed by Nov 1st each year to ensure they come back in time for January 1st.