Wish I'd seen this one earlier.
Here's the thing. She may be unhappy in her marriage. She may even have feelings for you. But she may not be anywhere NEAR ready to think of herself as a "lesbian", let alone come out of the closet to her husband, friends, family, etc etc.
Curious what makes you think she's 100% lesbian, as opposed to bisexual? Would that matter to you? I realise you've been friends awhile and you know her fairly well, but hey, none of my friends knew I was gay!
Yes, you should definitely proceed with caution...but you also shouldn't get your hopes up. Because with due respect to Katie (and much congrats!), her story really is the extreme exception and not the rule. Reality is far messier. And even if she does come out, she may well decide to stay married and faithful to her husband despite her feelings. Everyone makes their choices.
Really, what you are proposing is to have an affair with her. That's disrespectful to her husband, not matter what her sexuality might be. Wait til she comes out and decides what to do (if ever), then make your move.