I would start by talking to your friend. Start by telling her that you care for her a lot and they’re are going to be people that will like her but you like “Sarah” (made up a name for the bi friend because this’ll get tedious :P), and it’s rude for her to come in between that. Hopefully she’ll understand (it might take her time to get over so keep comforting her and make sure she knows that you care). Then to tell Sarah, you could just sneak something into a conversation hinting that you like girls. Good luck <3