best friends ex, should I? Options

[potatoespies]

My best friend is in a new relationship and I asked her if it would be okay if I pursued anything with her exboyfriend.
She said it was fine.
They only dated for a month a year ago and now they're really great friends, so I feel like it's okay.
But there's the rule of "never date a friends ex" so I'm scared.
Especially 'cause when she gets in a bad mood she'll laugh and joke about getting back with him (even though she hated dating him, she was too mellow for his craziness).

Should I pursue anything?
I don't want to make her feel bad, but I really do like him.

[superficial]

No! You never go out with your best friend's ex boyfriend. That's just a rule that everyone needs to follow in life. Especially your best friend's. I suggest that you don't try to pursue anything because deep down inside your best friend will feel a bit uncomfortable. Never get too close to your best friend's boyfriends in which you'll start having intimate feelings for them. So, no, don't pursue anything. It'll definitely hurt your friend's feelings even if she says it's fine.

[weed]

No! You never go out with your best friend's ex boyfriend. That's just a rule that everyone needs to follow in life. Especially your best friend's.

STEP AWAY FROM THE EX. I REPEAT STEP AWAY FROM THE EX. no matter how many times she agrees and approves, and no matter how many other guys shes dated. Dont go there. TRUST me, thats just like common law. Specially if she's your bestie. Deeeeeep down inside she might still have feelings for this guy and not know it, seeing you two together or finding out you two were fooling around might trigger emotions she didnt even know she had. trust me.

in the end you'll do what you please but for your sake and the sake of your friendship... just dont.

yes you should. he has a dick right? if you want sex, then be with him. her trash, is your treasure.

He's crazy. Dont listen to him.