Be cautious about your GF's best friend , she might be a Muslim

I dated a girl in my college . She was a very sweet girl , particularly close to me and I thought of her as a best friend more than anything , she was a co-founder of sorts of a startup I own , I used to discuss everything with her , she was a Brahmin girl . Everything seemed fine until one day I found out that she had a best friend who had converted back to Hinduism after marrying a Christian , but the girl was originally Muslim , my ex said , "oh , it doesn't matter whether she's Muslim or Christian etc" . Being a Brahmin myself , I really disliked this , and I told her , "it matters" . But she didn't understand . She continued to be friends with that girl . And I really didn't want to be associated with Muslims , so I broke up with her . And I think its a good thing I did .