In That Was Then, This is Now, Bryon cannot accept Mark dealing drugs.
Over the course of the narrative, Bryon's maturation makes him different than Mark. This is seen in how he gets a job and his emotional depth over Charlie's death. It is most evident in his involvement with Cathy. As a result of Bryon's emotional commitment to Cathy, he helps to find her brother, M&M. When Bryon sees him crashing from a very bad LSD trip and recognizes that the drug use has caused him to never be the same, Bryon possesses an emotional intricacy that Mark lacks.
Upon finding drugs in Mark's private stash and making the connection to M&M, Bryon recognizes the need to take action. He cannot accept Mark distributing drugs and ruining people's lives as a result. It is clear that this is the line that Bryon draws and that Mark has crossed. As a result, Bryon calls the police to have Mark arrested.
Bryon's maturation is fully displayed in how he rejects Mark's association with narcotics. He recognizes that Mark is incapable of distinguishing between right and wrong. He accepts that his moral compass is different than Mark's. This causes him to act. In opposition to Mark dealing drugs, Bryon firmly demonstrates what he cannot accept.