I wonder was the staff member just being rigid with the rules they had been taught. Perhaps the need for a bank statement is in case the customer doesn't have the bic/iban. Despite the fact your brother had his details the staff member may have just been like 'computer says no'. I worked in a bank and was asked to cash a cheque one day. I asked the customer for ID and they didn't have any and said they didn't have an account. I sent her on her way as we were told a person needed ID or an account. The next morning we were all called together and asked who had served this girl. I put my hand up and was ate! Turns out she was a daughter of a big bank client. If she had bothered to tell me I'd have cashed the bloody cheque but she ran off to tell daddy about the bad service instead. To this day (many years later) I still feel annoyed about it as I was just doing my job. The branch sent her vouchers for a local shop as an apology.
Anyway so maybe your brothers cashier was a gobshite like me