You can’t really expect to find your account’s stealer unless you have some legal authority. There are all sorts of middlemen from all around the world in acquiring the accounts and selling them on forums. Finding the person who stole your friends" account is like finding the same thief that stole from you in a holiday trip in another country, selling their stolen goods.
Speaking of; you could research where some database leaks are dumped to find out where these deals are happening. Can’t link any bc of instance rules.