From the payer's point of view, paying out a salary to a third party
bank account is generally not recommended but as long as there is authorisation, it shouldn't be a problem from a purely legal perspective (although check with a local lawyer).
It's possible that the payer's bank will ask for supporting documents, but this depends quite a bit on where the recipient bank is and how large the transactions are relative to the overall account turnover. A salary payment of 4,000 USD/EUR against an account balance of tens of thousands or annual revenues of hundreds of thousands isn't as likely to trigger an investigation as a 40,000 USD/EUR payment against the same overall financial profile.
Your mother's bank might start asking questions and want to see proof of source of income (such as employment agreement, payslips, proof of taxes being deducted). The risk of this depends on the magnitude and frequency of the income relative to your mother's history with the bank and current financial situation.
Another consideration would be if your mother is retired, receiving benefits, or something similar. Those are often conditioned upon the person's financial situation, which salary payments likely could affect.
And depending on amounts, flags may still be raised with the banks if transfers are made from your mother's account to your own account. It's unlikely anyone will care if it's just small amounts but if it becomes high amounts, especially over time, that can become something the bank starts digging into.
