There is only one reason to do this. Is this story accurate?
Most banks here have a daily limit for personal savings accounts. It varies from £500 to £3000 on demand in cash.
If you want more, you have to clear it with the bank approx one week ahead. It's not hugely sinister - banks try not to carry too much cash at any one time, to reduce the risk of robbery. It also prevents anyone draining your account if they find your passbook in the street.
He should have asked for a counter cheque made payable to his mother - there are no limits on those, other than the limit of your assets.
Just to clarify - the previous withdrawl. If he walked in at close of business and asked for £10,000 (for example) - if the bank has it and he can prove who he is, they'd most likely let him have it. Once it is logged out of the bank, they are no longer on the hook for it if the safe comes up short or gets removed.