As-Salamu alaykum my friend has said that she's currently on her period and knows that after 15 days she needs to do her ghusl but she forgot to write down when she started her period so she doesnt know when itll be 15 days since she started her period (She has PCOS) What should she do?

Ustadh AbdulAziz al-Haqqan

She does her best to investigate and take her precautions regarding the period. When it is most probably 15 days after she has done her investigations and taken her precautions, then Alhamdulillah, she can make ghusl and start praying and fasting and so on.