Years ago there were people who couldnt open Bank accounts so this request was a common one. Nowadays everyone can get some kind of Bank account so that shouldnt be the reason - sounds like its just choice. I would suggest its paid to the employee direct as a better audit trail. He can then pay his partner.
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Joanne
Winner of Bookkeeper of the Year 2015, 2016 & 2017
Thoughts are my own/not to be regarded as official advice,which should be sought from a suitably qualified Accountant.
You should check out answers with reference to the legal position
I have written a statement that they have signed stating their wishes to have the salary paid elsewhere and that we need 10 days notice to alter this condition so hopefully covered if HMRC question this with regard to PAYE/NI.