I have had the impression that you are not overly subject to emotional reactions, so given the fact that you had these definite alarm bells, I would suggest that they did their job.
In terms of the red flag, it was more that what he was doing didn't seem logical. I pretty much know she
wasn't right for him but I think a part of him, maybe subconsciously felt that someone else might
be right for a monogamous relationship. He says that the reason for him closing the relationship he is in now is because he can't tolerate her seeing other people (implying he still wants to himself) but obviously he can't want her to be monogamous whilst he does the poly thing. Yeah, I feel mislead and stuff but he isn't a bad person or anything; he thought he knew what he wanted and it turns out he didn't. Would I be confident if I were her about his sudden lifestyle change? No. Simply because it seems, if he was being honest, that he just fears being replaced/displaced in her life rather than genuinely not wanting to have relations with other people. If I were her, I'd be concerned that after a while, the urge to be with others would overwhelm him and he might do something unethical. But I'm not her and it's not my problem.