Do I believe my boyfriend or some girl?

Back in October I had gotten a text message from a friend explaining how a girl had told her she slept with my boyfriend. We have been together for 2 years and ever since this came out I am having a hard time believing him. He denies that he had anything to do with her. When this came out she had this happened in October when he was at his friends and when I confronted her she said it happened in April. She also was dating my boyfriend’s best friend. They were together before and after this so called relationship. I do not know what to believe. This girl had changed the story multiple times and told her boyfriend how funny this was. I don’t want to think he did this and I am not sure what to do. Help!!

Do I believe my boyfriend or some girl?
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