Friend Code?

If I like a guy that my friend used to, but claims that she doesn’t anymore, is it technically OK if i went ahead and went out with him? She even said it was fine and that i should, but you know how girls are. We say one thing, mean the complete opposite. She’s a no-BS kinda gal though, so I’m just curious on everyone’s thoughts. Like, what are exceptions to this rule, what are absolute NO-NOs. Etc. He REALLY likes me I can tell. And he doesn’t like her.
Thanks!