It might depend on what else I knew about the situation. I'm not exactly a duelist. If it was just a memory charm and not a curse or something and I didn't think the wizard would escalate to other kinds of violence, I might try getting physically in the way and try to talk them out of it, since I know they can't affect me. If I thought they were doing it illegally instead of just unethically, I might try to get a sensible grownup, like Mrs. Stark or Healer Song or one of the good teachers. If I thought I could talk an elf into helping without the elf getting hurt I might call an elf. If it was really bad I might ask an elf to get Feral for me again, but I'd have to be pretty sure the wizard needed to be dead to prevent something worse from happening. I don't know anything about whether I can block bad spells with my Patronus or something; I should experiment with it.