Tristan and Helen...One night stand?
Yes, a one night stand between Tris and the original Helen (Carol Drinkwater) seems quite possible in light of the circumstances at the time. To start with, they liked each other a great deal and were constantly kidding each other about why Tris didn't marry Helen before James met her. In the episode "Fair Means and Fowl," she takes his arm as she and Tris walk from the chicken hutch, where the Guinea fowl are, to the house.
We know from the last episode of season 3 that Tris remains at Skeldale House while Sigfried and James go off to war. Yes, I can easily imagine some cold, winter night when Mrs. Hall is perhaps away for the weekend. Helen is sad and lonely and she and Tris have had a pleasant dinner and consumed some of the fine wines that we know Sigfried stored in the locked dining room in the Christmas episode of season 2. A little wine, sitting on the couch in front of a warm fire, the lights down low, the two of them alone. Well, let your imagination run wild from here on.