Hi, I have a 5-month old and she has recently started rolling in her cot. During the night, she has rolled and gotten her arm stuck down the side between the slats. Just wondering what is recommended in this situation as I'm worried about her injuring her arm. Thank you.

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.
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

Once baby is rolling, continue to place her on her back for sleep & let her find her position of comfort if she is rolling well both ways.
How does she go with tummy time when awake?
Does she hold her head well & reach for toys?
Is she able to roll both ways?
Babies need arms free & the space of a cot to roll well.

Did she wake & cry when her arm was stuck through the slats?
It is not unusual for babies to get limbs stuck in the slats of the cot, at times, especially when they are “new to rolling”. Most stop doing this & sleeping bags tends to reduce the risk of legs getting stuck.

United States Consumer Product Safety Commission data has shown that infants suffer essentially no long term injuries from contact with cot slats