In their imagined conversation, why does Mariela say that she and Martin stopped being friends?

In the imagined conversation, Mariela recounts how she and Martin stopped being friends: Martin bought Mariela a bathing suit, hoping they could go swimming. Sensitive about her weight, Mariela lashed out, accusing Martin of being poor. Martin then accused Mariela’s father of being a “crook” (82).... then, he left.