Should I warn colleagues of another colleague's sexism?
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.