(im 16 btw)

A1: Direct Approach:
- Me: excuse me, I need a female opinion on something, it'll only take a minute..
- Hb: ok
- Me: well, my bestfriend has a girlfriend ok? and they've been going out for like 3 months, the girl is also one of my friends, but at a party we went to last night, he met some other girl and they made out and exchanged numbers and arranged a date, so what should i do now? should i tell her what he's doing? i already told him about this and he said "idc, tell her I already have another girlfriend."
- Hb: (if she say's don't tell her)
- Me: "lol, wow(shocked) you would let your friend be cheated on?" wow.
- Hb: (tell her)
- Me: Wow, you would squeal on your bestfriend? that's horrible.
- Hb:(idk)
- Me: Wow, you don't get around much do you?

That's a long opener - and all the talk of cheating may put you target in negative mindset.

Also, the answers you've picked to whatever she says are the opposite of her suggestion, so she has to defend herself right off the bat. It's a lot harder to make a connection with someone by disagreeing with them.

Instead try to agree and amplify, that way you are on the same side of the debate.

Never say excuse me and sorry to start off. Jump right in it. Give yourself a time constraint. It will take a minute only (and presumably then you will go do something else)
You grounded your opener by saying the reason why you ask.
The topic I think will definately get people talking, i will just try and get off the opener very quickly. You would do well to field-test it and tell us how it worked.

i will never talk about cheating to a woman im trying to pick up. when a girl thinks about cheating she thinks on how much A$$holes guys can be

and the story is to long remember to "KEEP IT SHORT AND SIMPLE" (aka K.I.S.S)