Hi Solecoria,
Don't worry. At 8 months she should be able to roll backwards and forwards on her own without any trouble, and can turn her head to breath. The reason they recommend not to put babies on the tummies, is that young babies cannot lift or turn their heads to breath, and therefore can suffocate. My 9 month old son has been sleeping on his tummy now for about 2 months. If you put him on his back, he will roll over onto his tummy within minutes, usually with his backside stuck high in the air. Must be a comfortable way for them to sleep. As long as she is sleeping, I would leave her as she feels comfortable.
Tracey
Mother of 3, Qld