Please provide screenshots of the complete chat with the accused via your Friend Chat Message history. Please do this in a web browser and in at least one of the screenshots hover your mouse over the accused's name so that their profile URL shows in the bottom corner of the browser. All screenshots need to be uncropped and unedited.
You can view your Friend Chat Message history by going logging into Steam and going to,
Steam Support > My Account > Data Related to your Steam Account > Friend Chat Messages