Trying to discredit a man's argument by pointing out his associations would indeed be a Guilt By Association logical fallacy. However, as a Christian, pointing out that his associations with heretics and compromisers are unwise and lend credibility to the message of the heretics and compromisers is NOT a logical fallacy.
It is not a fallacy to point out the foolishness of lending credibility to false teachers for this very reason: *BECAUSE* PEOPLE as a rule have a tendency to think illogically about associations. i.e. Horton, a sound biblical teacher, doesn’t speak against Warren clearly so Warren must be not so bad. It’s just the facts of life. We work with it. It's just the understandable shortcoming of human communication. I agree it is impossible to stop people from thinking irrationally in all cases, but what have we got to lose by just refusing to associate with heretics? Status, perhaps? We want to look big and welcoming and have the praise of men? Sometimes. When we have to say no, we look narrow minded, and we don't want to be given that label.
I would also contend that it sounds from what Horton has said publicly about Warren, that Horton doesn't actually consider Warren a heretic, as Chris does. This is another problem that figures into his being seen with him, and strangely enough, Chris has not mentioned anything about this.
I agree there are certain levels in which we can work with people of different mindsets. Ingrid does too. But everyone has a different threshold for cooperation, because we are all imperfect. Warnings against cooperation are a lot like warnings that you shouldn't be seen alone with a woman not your spouse. Why? Because of the message it conveys. It doesn't mean you or the warn-er hate or reject that woman or that they hate or reject you, nor does it mean that they are calling you an adulterer.
How many people have seen men unintentionally respond to a flirtatious woman? Neither one of them have to have any ulterior motive for this to be seen as improper and to make people uncomfortable. They both just have to be naive as to how it looks.
As far as the "bridger" concept goes:
"For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ."...
..."These are hidden reefs at your love feasts, as they feast with you without fear, shepherds feeding themselves; waterless clouds, swept along by winds; fruitless trees in late autumn, twice dead, uprooted; wild waves of the sea, casting up the foam of their own shame; wandering stars, for whom the gloom of utter darkness has been reserved forever." --Jude 4,12,13