my 8 month old granddaughter has just started to sleep face planting down in her cot. She is able to roll from back to tummy but has only rolled from tummy to back a few times....her development is a little delayed due to a difficult birth, she swolled the meconium at birth and had to be resuscitated. A nurse has told my daughter she should be fine, she got herself there...My daughter and husband are constantly waking to change her position.... All i have suggested is to increase her tummy time to strengthen her neck. any other advise.

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, & some face plant at times.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

It is important that she is now sleeping in a cot & not being wrapped.  Baby needs arms free & the space of a cot to roll well.
Always continue to place baby on back for sleep.

Yes, it is important to increase tummy time when awake to build upper body strength.
As she is faceplanting at times, ensure the mattress is firm & their is noting loose or padded in her sleep environment.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over