Should I warn colleagues of another colleague's sexism? Over the years, I have had experiences at some conferences where more senior faculty members have been—shall we say—less than appropriate to women in the field. The same can be said about colleagues being heterosexist and/or racist, but I digress. Sometimes, these are isolated incidents and I didn’t feel comfortable having the confrontation at that point in my career so I let it slide. I am in a field that has a serious gender inequity problem and I am a junior member of the field.

Flash forward a few years and a colleague (of the gender/race/sexuality that the other person disparaged) is going to a conference with the same person. In my field, the conferences are small, so there's little chance they will not interact. 

Does one forewarn one’s colleague about such a faculty member? Part of me desires to forewarn, but part of me worries that it will just add drama to the community, preconceived notions to the colleague's experience in the conference, and possibly unnecessary dread during the time interacting with the faculty member. What if the faculty member has gotten better about this? What if it gets back to the faculty member that I’ve said that they’re sexist?

If one doesn’t tell a colleague and then experiences said sexism, then does one confirm their experience about that faculty member to help one’s colleague process that experience?
If this person is "merely" boorish, I think you're best off letting your colleague experience and handle the situation themselves.  Presumably they're also professional adults who can take care of themselves, and as you said, it's possible the other person has gotten better about it, so maybe it's a non-issue.  But more importantly, since you're all in the same field (which you suggested was small), being the one to spread the word like this (even if it's justified) can have repercussions for you.  Whether you want to take that on probably depends on just how bad the person is and on how comfortable you are with your position within the field.

On the other hand, if you think this other faculty actually rises to the level of a danger - anything from sabotaging your colleagues work or relations, to not being safe to have drinks around - you should absolutely warn this person.

Obviously there's a judgment call here.  @whrrgarbl suggested it depends on how friendly you are with your colleague too, and I think that's true around the margins of this question.  In a totally general way though, I think when you're contemplating meddling in affairs between two other people that the default answer should be no.  Especially in a professional setting.