So are you guys in graduate school? Wow sleeping with your best friends GF, thats pretty low, I mean it sounds like she still wants to get with this guy your interested in. No one wants to be alone so its a bit of a tense situation, this woman is not going to dump this guys friend until he commits to her but he wants to get with you? I guess alot of the creepyness hinges on how this guys best friend treats his GF otherwise its unlikely she would cheat.
Why would you say she is the woman to avoid in proverbs...because she is not you? Some people are not comfortable letting go of the bird in the hand for the one in the bush until the one in the bush is in their second hand then they can let go of the other one and not be empty handed, espeically if your not particularly fortunate finding someone decent in the past, it can get really sticky. People who have no issues finding new partners dont usually do that, they will just straight up dump you.
Another thing is he could be looking at you as a hard nut to crack, when his hormones start raging he is not going to want to have awkward dates with no sex and a courtship that lasts until he has one foot in the grave, he may really like you but that could be a major black mark on his list of your traits. I highly doubt that this other woman is just a mentor, he may not be having sex with her but he is thinking about it, I would be surprised if he has willingly not had sex in a year (maybe unwillingly). The fact that this woman is his best friends GF is the real crux of this whole situation, maybe not for you but from a 3rd party stand point thats the only thing that really separates this situation from a guy thats just playing the field to get what he really wants.
That is the crux of waiting till "marriage" for sex with outragious courtship time lenghts, some guys dont really view that as a real relationship, your basicly just friends having coffee or whatever so if some woman comes along that suits him better you may be starting over and its not really cheating because you really have nothing invested in each other other than some time drinking coffee. These super long engagements and courtships can really hose up your chances at finding someone.
Maybe just break it off and join a church singles group where the intention is to meet someone and get married. Anyone who is willing to just sit and drink coffee with no sex for months and months or years is looking for a real relationship on the side at the same time, I gaurentee it, unless they are a total putz and then you have to really question their masculinity.