My sister alhamdullilah is becoming closer to islam and does not want to miss out on being forgiven on the day of arafah and the great reward however she is ineligible to part take in fasting or prayer on this day (menstruation ) . Should I ask allah for repentance for her or can she just make dua.

Ustadh AbdulAziz al-Haqqan

She can make dua; that's not a problem. If somebody really wanted to perform an act of worship but was unable to due to a reason out of their control, they get rewarded for it either way.