what do i do when my best friend (who is now in a relationship) refuses for me to talk to a guy that i'm already friends with because she claims to have had a crush on him in the past (which i never knew about)?

There's no bad thing on talking to the guy, You're just friends. And besides she has a boyfriend now so what? 😂

Something sounds fishy. She is in a relationship (I'm assuming with someone other than the guy u want to talk to) and she doesn't want u to speak with said guy? She doesn't own him or u. Go behind her back and do it anyway.