Succession: What is the way out of this?

My friend’s father expired.
Has some (probably LARGE) equity holding in his Demat account.
He has two Sons.
Both are US citizens.
There’s no other legal heir in India.
Now, what is the way out to get to this holding in his son’s accounts?

Normally most demat accounts will have Nominee registered . if it is so the process can be easy but not sure about nominee/s as US citizen ? you need to consult a lawyer who expertise in such aspect .