I personally believe sexuality is often on a spectrum. There are sometimes, on some occasions, exceptions to most everyone's usual sexual orientation.
If you value her friendship, I'd be honest. But, not a creeper! "Hey, if you ever want to change your luck, and if I happened to be attractive to you, I'd be willing. But, even though I think you're smoking hot, I'd rather be your friend than your lover, if I have to choose.
".
And, then don't creep. If she can't trust that every time she has four margaritas you won't start looking sharky at her or you can't keep your eyes off her tits...yeah, never gonna happen.
But.....if you are a friend she likes, trusts and respects and knows you feel the same about her...... And, then she ever does feel like changing her luck for a night..... Well, as opposed to an oaf at a bar, she might choose her bro!