If she is receiving Maternity Allowance, she can work for a total of ten days without it being affected. If it�s Statutory Maternity Pay (SMP) she is allowed to work ten days for the employer paying the SMP. However, if she does any work for another employer, the SMP should stop.
You will not be telt anything about another person�s Benefits so there is no point in asking DWP or HMRC what she's getting. If you have any suspicions you can �phone 0800 854 440 between 7.00 am and 11.00 pm