Ask HN: How can my friend recover his Instagram account from a scammer?

An acquaintance of mine had his Instagram account stolen by an active hacker; the guy is still using it and regularly shows up online.

Efforts by my friend to recover it don't seem to have gotten anywhere. What would be the process by which he can regain control over the account? I could report the account, but it would just lead to a suspension, not a recovery.