If someone was about to say a story that was Lashan Hara, you made a comment, which in essence made him continue his story (a follow up question ). Now, you know 100% if you didn’t make this follow up, he would’ve still said it anyway. Are you responsible? Do you have to ask mechila?
If I understand your question correctly, you mean to say that the lashon hora would have been said regardless of what you said. In that case, essentially you didn’t cause his speaking lashon hora, and you were not over on lifnei iver, nor do you have ask anyone mechila regarding the other person’s speaking lashon hora. It is needless to say, that you shouldn’t be mekabel the lashon hora.