I have been in this situation many times, its best to not say anything until she's single imo
Never been in this situation but this is the answer. If she did things that made it obvious she liked you (from the OP it looks like you really are just a friend) than I would actually say go for it
It's all about reading this situation. Best thing to do in situations like this is look at the picture from different perspectives and not just your own
Here's a perspective
My S.O. of 17 years was in a serious relationship with my best friend when I told her flat out that after all the time we were spending together, a platonic relationship wasn't going to work for me. That was absolutely, positively the right thing to do. In fact, it was probably best for everyone in the long run - once the dust settled everyone in that love triangle had a better relationship with each other afterward than before. The friend in that arrangement went on to get married to someone he's crazy about and have kids. Not in that order, and not the same women, but hey, it worked out.