She sounds very sad. It's nothing to do with him and everything to do with her own insecurities. He need not take that one... if he can I hope he is deflecting that energy and giving it back to her. It isn't his to take on. Its not his fault if she is lost.
It sounds like you don't have Veto so much as you give him your opinion. That is a bit different. Veto is putting your foot down and saying no entirely. Him having a hard time judging means that you offer him some sound advice on situations that he trusts. That is a good thing. It can be hard to not turn it into a veto however, as it sounds like it drives you crazy.
I had a situation like yours at one point. I had to be very firm with PN that the woman he was madly in love with and NREing all over the place didn't love him and was using him for sex. He didn't see it and took his attention away from me and his boy as a result. I eventually told him that I thought she had to go and he agreed and called it off. It broke his heart and he was angry with me and disappointed... it turned out that he had done the right thing because the first thing she said back was "oh that's too bad, I won't be able to have sex with you any more now." It was that statement that turned it around for him and he has been grateful ever since that he trusted me. It built our trust from there on in. He gets major rose coloured glasses and is hopeless with NRE issues! To the point where he is determined not to date again and is mono to me at the moment.
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