I am a Master's student in a field of the humanities. Half a year ago I was invited to an informal colloquium of mostly doctorands and post-docs who work at the institute I'm studying at. One participant's draft for an essay was the topic of the first meeting.
On working through that draft the evening before, I found five sentences in total that were obviously copied from online sources (blogs, wikis, etc.) without any citation. Due to stress I decided to postpone the decision on what to do with my findings and did not attend the colloquium at all.
To date, I can find no evidence online that the essay has been made available to the public and I do not know if it was ever supposed to be. Its form though is clearly that of an essay (not a sketch or an unstructured gathering of ideas), so that I can not think of any circumstance under which this would not qualify as straight-out plagiarism.
The author is still working at the institute and holding classes.
What is the professional way to handle my findings?