Ladies, how would you respond if a girl reached out to you warning you about a guy?

There's a girl I know who has had an issue with a manipulative guy, who had been trying to get with her. Verbally abusing her. Just treating her horrible. He had also told her he got with another girl to make her jealous, and left her after 3 days when he didn't get the reaction he wanted out of her. Well, after several months of his foolishness, he started dating a girl. She wants to reach out to inform that girl of his behavior, as well as the fact that he called her and was being flirtatious with her on that call.

So how would you respond?

Ladies, how would you respond if a girl reached out to you warning you about a guy?
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