In all my reports, albeit for harassment directed at others, Twitter has never responded to a single one.
I’ve been increasingly disturbed by the overall lack of civility online, but even more disturbed…
Lisa Angela

As far as I know, Twitter never responds to or acts upon reports that another user is being harassed. They only respond to reports when the target is the person reporting it.

I once reported a user for targeted harassment, and included some of their harassment of other users in my report. My report was initially rejected (“not abuse”) but after a Twitter employee who saw the abuse offered to escalate the case, the user was required to delete their tweets at me — but their harassment of other users, including the use of racial slurs and suggestions of self-harm and other forms of violence, were allowed to stay up.