Hi, my baby has been sleeping on her belly every night for two months now and she can roll from her back to belly and belly to back, I walked in there tonight and she had her head face down in the mattress but seemed to be breathing fine, is this okay? If not how do I stop it?

How old is your baby?
Is she in a cot (not bassinet) & no longer wrapped/swaddled?
Babies need arms free & space of a cot to roll well.
Always place baby on back for sleep (even when rolling)

Ensure the mattress is firm & flat with nothing loose or soft in the cot, especially as she is face planting on occasions.
Did you try to turn her face to the side? What happened?
Most babies are rolling around 4 - 5 months, some earlier, some later & some do face plant - this is where the firmness of the mattress is so important.
Rolling is a normal development stage, but it is still important to continue to provide a safe environment for baby to sleep.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

I am not aware of any you can do to stop baby from face planting.
Let me know the answers to questions above , as I may be able to provide more information.