A friend was fired after rejoining her firm after taking approved sick leaves taken in writing using a doctor's note. What to do now?

Hey Folks. I'm writing this as something unpleasant and disturbing has occurred to my friend at the workplace. Last month a friend of mine applied for paid/unpaid sick leaves of 12 days with a doctor's note for her treatment. After approval from HR and managers (on the rejoining date), they fired her without notice on the joining date without any explanation.

I wanted to understand what are the legal options available for this? I'm perplexed