Is it possible to bypass iCloud password so I can restore factory settings on a stolen phone I recently recovered

My eldest son stole an old iPhone that used to belong to my daughter. He evidently guessed her password (she was 13 when she owned the phone and used the same 1 or 2 passwords across the board). I’m trying to restore the phone so my mother can use it, but I can’t get past the thief’s iCloud password. He’s in his mid-30’s and a drug addict, btw, lest you think my language too harsh. I’ve no way of contacting him. Is there any remedy?