I presume from your question that you friend is also employed in this "casual" job and is paid as an employee.
The answer to your question is that her entitlement to SMP with her principal employer is not affected if she is also employed in the "qualifying week" for the second or any other employers. Only if she starts a new job after the qualifying week and continues to work in that job after her baby is born does she lose entitlement to SMP.
The qualifying week is the Sunday to Saturday week that starts on the Sunday that is 15 weeks before the Sunday of the week in which her MATB1 certificate states that her baby is due. It is the same week that her main employer decides on her entitlement, or otherwise, to SMP.
Ian Congreave retired from his payroll and HR work at the end of November 2011. He founded the Forum in 2004, when it was known as the PayPerShop Forum. Ian would like to thank the many Forum members who have asked and answered hundreds of questions and helped to make the Forum a valuable resource to the payroll community.