It strikes me that the Garden provided an environment where the student genuinely wanted to follow the path of Epicurus, and the teacher genuinely wanted to help the student. Some students may have been more amenable to this correction than others, but that didn't stop the teacher from administering the medicine.
Even if at times there was frank speech between two students, I can imagine that it was based on the teachings (it wouldn't be random). It seems that between students the frank speech would have been cited/referenced to specific teachings which were found in Epicurus' books.
Some questions to consider (and if anyone wants to reply):
At what point does frank speech come across as rude, harsh, or insensitive? Is the phrase "frank speech" ever used to justify harsh speech? What is harsh speech and when should we avoid it? And, what kinds of word choices appear to have a domineering, shaming, or power-over (controlling) attitude? When is frank speech just an expression of anger and a kind of punishment?