Complex sentence: He and Andrea develop a romantic relationship as she settles in to the town; however, it is at the expense of her friendship with Michonne, who ultimately decides to abandon her and leave, eventually meeting up with Rick's group.
Split into simpler sentences: He and Andrea develop a romantic relationship as she settles in to the town; however, it is at the expense of her friendship with Michonne. Andrea's decision to remain in town prompts Michonne to abandon her and leave, eventually meeting up with Rick's group.