At least in my testing on the latest version of the plugin and Moodle 3.5 and 3.6 that will still mark the activity as complete, but completed (did not achieve pass grade). So if course completion is tied to activity completion that will still mark as complete despite the no show. In some plugins there is an option to restriction completion to a passing grade, but F2F doesn't support that.
So in this use case I would set the required course grade to 75 or 100 for course completion rather than rely on activity completion, or if there are other activities I would use a grade restriction instead of an activity completion to restrict access to the later activities.
Jeremy