[Zhgrlpil]

Would it follow that this should only be done if there is some benefit intended to result from relating said activities, or is it simply permissible to talk about the activities of an open sinner for no good reason?

To talk with no good reason is a sin in itself.
Let us separate that from the present discussion.

So let us take the situation where you get up and tell your friend that you got to buy milk from a particular shop.
Your friend says that do not do that - his milk is dilute. In this case your friend stands absolved of gheebah.