Asking on befalf of my 5 month old grandchild who likes to sleep on her tummy? Everytime she is put down on her back she rolls to her tummy? Smoke free environment with nothing in her cot. She sleeps in a sleeping sack and with a dummy. She can roll back to tummy but not reversed? How do I prevent her from sleeping face down? I could turn her everytime but she still rolls to her tummy. Any suggestions or do you think she will be ok?

Most babies will be rolling over by 4- 5 months…...some earlier, some later. 
This is normal as babies grow & become stronger.                         
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

Does the sleeping bag she is using allow her arms to be out - ie not contained like a swaddle?
Babies who are rolling need the space of a cot & arms free to roll safely.
How does she go with tummy time when awake - holding her head, reaching for objects?

Always continue to place baby on back for sleep.

Red Nose has the following general information now that she is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

**As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over