I want to publish in an open group in Facebook and make sure that one of my friends won't see this posts.
How can I do this? If I will block this friend, will he also be able to see my posts there?
(Maybe he liked this group and maybe not.)
If it's an open group and he/she subscribes to it, yes he will see the posts. Blocking the person is an option.
The correct option is not writing posts behind friends' backs