I'm dating a guy named Ethan & my friend Sandy likes she doesn't know me & Ethan are dating she tries to flirt with him & hug him she's always telling me how cute he is it bothers me what should I do

You say she is your best friend.
You say she doesn't know.
Best friends know everything.
So either tell her or she isn't your best friend and it doesn't matter what she says and does.

You tell her he is your boyfriend and to chill with the flirting.
I don't understand what the problem is. He's your boyfriend, so what if she tells you he's cute. How does your boyfriend react in all this.
How old are you?