The best example of a respectful way to warn an abusive customer is:
"Ma’am, I know you have had a difficult time, and I’m very sorry you had to go through that. I do want to help you and think I may have a solution that works. Can you help me fix your issue now?"
This response acknowledges the customer's frustration while remaining calm and polite, showing a willingness to help while implicitly setting a boundary against abusive behavior.