MC behaviour with the other isekaied is retarded, if he suspects the guy he should share what he thiks with the girl, of course the chance of her thinking he is talking badly of that party leader because he doesn't want her to join them would be high so using his maturity he should bring it up as things he gained through experience, that he just distrust people and that she should be careful of some signs that could match his fears, so he wouldn't say she is doing the wrong move but would let teh choice to her, of course what he says would make her paranoid and as it seems like he did get some good hints on that guy it would go like that and she would leave before is too late, but now I believe she wil lstick for a while and get fucked like a harem girl or get sold when she refuses to.