What to do if my friend thinks I’m lying about dating someone?

I just started dating another 15 year old that I race with. He doesn’t go to our school, and my friend doesn’t know him. I told my friend I started dating him, and apparently he texted her on Instagram and asked if she was the one I was friends with and asked her for advice on dating me. He made the Instagram account for the purpose of texting me so people aren’t reading our messages to each other and he made the account like an hour before he texted her. She doesn’t believe me and thinks I made a fake account pretending to be him, because he was only following me, he had no other followers, no bio, no profile pic, and no posts. I feel like she should know I wouldn’t lie to her. She also told one of our mutual friends that I have a boyfriend now but that she didn’t know she believed me. And now the two of them are talking badly about my boyfriend and I and they said about how bad his one caption on an Insta post was and how thy are concerned for his well-being. Whenever I try to talk to her about it and why she is accusing me of lying, she always blows me off and says she doesn’t care. How do I make her realize I’m telling the truth?
What to do if my friend thinks I’m lying about dating someone?
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