"She is playing."
Disclaimer: I am learning, too...
"Ona si hraje" seems to me to be fine. You might try reporting it as My Answer Should Be Accepted, and it may become an alternative translation.
As I understand it, "hrát si" means playing more or less on one's own, as a child or a pet playing with a toy, for example.. To play on a musical instrument would be "hrát na XXX."