Unacceptable unless like pp said and it is to close a bank account or other legal issues in which case is totally acceptable. For starters it would cost you money to obtain a death certificate, secondly it would be pretty confrontational and insensitive to ask the nearest and dearest to borrow the death certificate to show as proof for a missed appointment so you don't lose money, thirdly it is an appointment for behavioural issues? I would advise the reception staff they need to make their own appointment because that is just plain rude. And finally I would think a medical certificate from your gp would suffice if they really felt it were necessary or a funeral notice.
How insensitive and rude.
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