My little girl has been doing this for quite a few months now, initially I started rolling her back but it just became impossible and she would wake up every time. After a few weeks of sleepless nights I just gave in and let her sleep on her tummy. She's been absolutely fine. She has now learnt to roll both ways but before this, she would definitely let me know if she didn't want to be on her tummy anymore. I believe babies are born with a reflex to turn their heads to whichever side they can get the most air so I wouldn't worry about not being able to breathe as long as you don't have anything else in your babies sleep area like teddies/blankets ect it shouldn't be a problem x
My LG is 4 months old and has been rolling into her tummy for quite some time. She has not mastered rolling back onto her back yet but she loves sleeping on her tummy. I was quite nervous at first and was rolling her back into her back. She has a great neck support now and is holding her head very well so when on her tummy sleeping, she is moving her head sideways and sleeps with no problems x