I had a conversation with one of my homegirls while we were doing a group trip together. I ended up filling her in with the basics of dating someone. It's really easy to remember and requires only two steps and the last one might require some more effort.
The first thing is to have enough confidence. The second thing is to go ask her out. The last requirement is that she needs to be the right person in order for her to say yes and things to work out in the end. The last requirement might take a while to get it to work, but by playing a numbers game and approaching all the women you can, you will logically increase your chances.
Honestly, I think there's really no reason to try to steal a girl who isn't that interested and says she's already taken! It's going to be a waste of time, plus there's a greater chance you will find out a deal breaker about her if she lets her guard down after being friends for several years which means all this time that can't be brought back was lost. It's pretty sad but it is still better to just move on, if you aren't interested in having any friendship.