SO I'm dating my close friend ex boyfriend ! It was love at first sight! We are deeply in love ! I don't know how to tell her that her ex and I are deeply in love ! Thing is she still loves him ! Please help me and tell me what's the best thing I should do right now !

Well if she's your real friend she won't care.if she's not your real friend she be a jerk to you it's your choice

That love at first sight, and deeply in love thing is, not only over rated, but seldom lasts, and are two very different things. So tell her the truth, whether you lose her friendship or not, or keep it to yourself, and continue to live a lie to her face. Those are your choices, and either way that's a lousy way to treat a best friend.

How old are you, this sounds like playground drama!

I meant your friend not your friends ex

There is no such thing as deeply in love this quickly. This is infatuation.
Only you can decide between this 'love' that probably won't last long and the lasting true friendship of a best friend, who now won't even be there for you when this guy ditches you like he ditched her, right?