Well, at first it was Kelly who made the decision not to tell Brenda. She definitely decided to tell her only once she was chosen and she knew she would at least have Dylan in case Brenda disowns her, so basically when she was on the safe side. Dylan was a jerk for thinking "honesty is not necessarily the best policy", but he did not put any pressure on Kelly to tell Brenda. Had Kelly wanted to tell Brenda, she could have done that numerous times not waiting for Dylan. And had she been a true friend to Brenda at that point and really chosen friendship over the guy, she would have done that. This way, she basically showed that from the start she teamed up with Dylan, not with Brenda.

Kelly made it seem as telling Brenda late was strictly Dylan's decision, which it was not. Even if it was, it was so stupid to listen to him. She turned into a weak little woman once she hooked up with him.

Dylan was sooo disrespectful to both of them telling Kelly to go tell Brenda if she likes, as if it was none of his business (hallway scene in Back in the High Life Again). It means he did not care about her feelings as well. He only went with her to admit infidelity to Brenda because Kelly blackmailed him with not having sex until they tell truth - together. Kelly realized she could not rely on him and that was one of roots of her insecurity in that relationship.