My daughter started rolling over onto her stomach during sleep at only 2 1/2 months old. At first I got really worried about it due to the SIDS risk, but I was told that if she can roll, it means her neck is strong enough that she can move her head to breathe easier during sleep. She also sleeps better and longer when she's on her tummy than on her back. Once I tried to swaddle her so that she wouldn't roll over, she rolled over anyway!
The boppy has a tag attached that states do not allow a baby to sleep in a boppy. It's probably better for the baby to sleep in a straight position - better for back development.