A baby in care actually sleeps on their tummy. When placed into the cot on their back, they will roll over back onto their tummy. Parents say she sleeps on tummy at home. Do we have to keep turning her back onto her back? Do 5min checks instead of 10 min? Thank you

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy. Some babies also face plant at times.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

How old is the baby you are asking about?
Can she roll both ways? - back to front & front to back.
Once baby can roll well, still always place baby on back & allow them to sleep in their position of comfort.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Is this baby being wrapped/swaddled?
Baby needs arms free (& the space of a cot) to roll well.
Continue to always place baby on back for sleep.

If a baby is rolling well, not swaddled & is placed on back for sleep, it should not be necessary to routinely increase sleep checks,
What does your Policy say?