The 28 days only applies to the date he submits his online application/pays for the visa... as this will be his official application date.
So, if his employer letter is dated 5th a November, that means he has until 3rd December to submit the online application.
His latest payslip and bank statement must also be dated no more than 28 days before the date he submits the online application.. so if he submits it tomorrow, December 1st, the latest payslip and bank statement must be dated no earlier than Nov 3rd and no later than Dec 1st.
He then has until his biometrics appointment to upload all his paperwork/documents.
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