When is it okay to date your friend's significant other?

My cousin informed me that my brother and her brother (let’s call him Joe) that they were having some issues with each other. I seriously didn’t want to get into it because it’s none of our business and I knew anything I say will be used against me since it’s concerning her brother and she’d think I’m taking sides. She’s very protective of him.

Anyways, Joe had been talking to a girl for about a month. Nothing serious, just talking until a week ago she said she thought things would be best if they were friends and she doesn’t see a relationship.
Ever since my brother was dumped by his girlfriend of 3 years he’s been kind of talking to every girl who gives him attention LOL So I guess Joe’s “girl” gave my brother some interest.
My cousin said that my brother asked Joe JOKINGLY if he could go out with her and Joe said he wouldn’t care.

To be honest, how do you JOKINGLY ask someone if they’d be okay with dating your “ex”?

Anyways, my brother went with her on a couple dates and Joe is now very hurt. My cousin started yelling at me because I said it was none of our business and thought I was taking my brother’s side even though I said it was messed up…but I guess she wanted me to say more because what I said wasn’t enough. Like I predicted, she got mad at me for not being mad enough I guess.

Anyways, it just made me wonder. When is it ever okay to date your best friend’s girlfriend if it was short term or long term? Is there an exception? My brother and Joe are very close.