My 3 months old baby always sleeps by his tummy? Is it ok to allow him to sleep at night like that ?

My gal was used to sleeping on her tummy since she was 2 mths old. Nanny told me that she won't get startled easily and will feel v secured. However, it is not advisable when baby knows how to turn their heads Coz they might suffocate (SID) if we are not careful. Despite that, my gal, now 5, still prefers to sleep on her tummy. Just that we need to be more careful especially at night.