I really like this guy but his x is a *****!(pardon my language lol)?

I have liked this guy for a long time and I think he is bi. I was going to ask him but recently his x gf has arrived here for a holiday for one term at my school. She is nasty, sy and the bitchiest girl I have ever met, I think she knows I like him and I'm afraid shes gonna tell him? I think she doesn't like me and is meaner to me becuase she can tell I'm gay(shes the only 1 that can in the whole school) and because she stills likes him? But I could be wrong. Does anyone have any ideas as to what I can do. His x is very gorgeous and is extremely popular already., so no one dislikes her except me, as all my friends are straight and secretly have obvious crushes on her. Can anyone tell me what to do? I want to ask him but she might tell the whole school about me.