Can't quite figure this one out. Andy is unusually passive and seemingly way too prepared to take a back seat to the new teacher friend (Frank) of Helen's. Helen sits closer than I would think appropriate in almost every scene while Frank though seemingly mild is fairly annoying as a kinda know-it-all.
It seems to me from the first night Helen brought Frank over, Andy should have stepped up more, for instance when sitting on the bench out front with Helen prior to the guitar scene, he didn't so much as put his arm along the bench behind her to indicate their relationship status. That being said, I think Helen was being disrespectful towards Andy in several instances.
From what I understand with reading of the Andy and Don biography book, there was a certain amount of sexual tension between Andy Griffith and Aneta Corsaut as they were into each other though he was married, so this may very well have played over into some of the series episodes.
Aunt Bea and Gomer are the only ones that seem to have any sense in this episode as to commenting in regards to how things are playing out. Especially Gomer in the scene at the café.