Dating my best friends cousin even tho they hate each other?

Ok so she has been my best friend for about 2 years and i met her cousin at a party but i also found out at that party is that her and her cousin dont get along AT ALL. But me and him were talking even mire than me and her did. And at one point he took my phone and hot my kik. And then we started texting. Then he asked me out and at this point i really liked him so i said yes but when i told my friend she got really mad. Is dating him not ok?