I'm going to date my best friend's ex
Don't you think you should talk it over with your best friend first?
That's a bad idea, I'm drunk, and I might get bored again soon without them
You should talk to your best friend about your feelings for their ex and get their input.  Be thoughtful and considerate of their feelings and be as transparent as possible when discussing your desire to date that person.
Nah, I'm going to stop talking to their ex when they get jealous.
You should think again before you do this. It could end your friendship to date her ex.