She still followed him away from everyone else and didn't warn/signal mc or anyone. She is now alone with a bad guy of unknown strength. And who knows how many of his allies are around.
Absolutly right, but then again one of her allies is missing. If she attacked him outright or somewhere outside, he might have just run away and they would have lost their only trace to her. Now if she would be absolutly sure MC would come following her the moment she went with the imposter, it would be fine. That's the only (but major) fault I see here, for the reasons you stated.
But goating him into leading her into his hideout, finding her missing new companion and running away, if things get bad, doesn't seem too bad. It will go bad though, when the imposter (and his still possibly existing allies) threaten to kill or manhandle the halfling girl. As a good aligned paladin at heart, that's a good way to extort her into surrendering and running nowhere.
And this is where I started to ask to myself, "Huh? No way! She can't possible be that smart, educated or wise in this matters, can she?" She attacked that guy,
before she found herself in such a situation. They already arived at a building. In case he lead her to the wrong place, her spoiling it now by attacking wouldn't change a thing about finding the girl anyway. But she doesn't have to face a possible hostage stituation outright and there is a good chance she acctually evaded it. If his allies come running, they most likely will do it without the hostage. And even in case someone stays by the hostage, they won't do anything to her, until they know what happened. Also the imposter was below her on the stairs and off guard, therefore had a good opportunity to attack. And if she has to run because he proves to be too much of a challenge, there is a good chance, she gets away, before he can catch up on her train of thought and start threatening to do things to the bard girl. (Of course, and that's the downside to it, then there also isn't someone there to stop him from doing so, to get back at paladin girl.)
That stupid and stupidly naive Paladin girl can't have figured all of that out on her own, can she. That would be acctual prove, that she is indeed not rotten in the brains.