I like my best friends ex gf, who she hates

I have a friend, lets call her amber, and she was dating this girl for a while, and she was really in love with her. Then the girl broke up w her and said things like they had never been actually dating, and it really hurt amber. Ofc she told me all about this, and I feel like I should hate her ex, lets call her Kat. Butttt Kat has been asking me for help about her sexuality/gender identity, and saying stuff like "you're so pretty" when she 'reacts' to my instagram stories. Aaaaand I'm lowkey catching feelings. But amber can never know, because I just know that she would hate me for being with, or having s crush on Kat. So either one of my best friends or a cute girlfriend. Great. I dont want to choose.