Hmm..so I didn't read the last half of your post.
1. Nobody I've known or seen has ever said "I just had to cheat on my partner because new person was SO MUCH HOTTER than my current partner. That's an attitude you really need to throw out if you want to have successful poly relationships, people cheat for lots of reasons, but unless its an episode of LAW & ORDER it's probably not because somebody was sexier than you. Lets say this girl was not attractive to you - does that make the situation untenable? Lets say nobody mentioned a placating threesome?
2. Maybe it got said in the second half. Poly = ethical. Wife's friend needs to know that it is honest and open from now on, and that you know what is going on.
3. Really? Best Friend can't handle confrontation or bad feelings cause she'll need to get stoned to deal with actual life. Well then - be upfront and honest and let her go get stoned to hide from her feelings...maybe you'll be surprised and find out she gets stoned cause she likes being high, and she is perfectly capable of being in a healthy relationship and communicating her wants and needs if everybody isn't lying or hedging.
I've imagined I've read the rest of your post - my response to the hypothetical reading is that no matter how much you're OK with your wife lying and cheating on you because you want to screw the hell out of her bf (or are really truly romantically in love with her) none of that matters if you can't sort out #1-3. You and your wife have a lot of shit to work out on your own front as it is. Agreeing to hot threesomes with this woman and your wife is totally different than being poly, especially lets say her bf rejects you and only wants to keep dating your wife while you just have unrequited love for her - that is something you should be prepared for if you want to go forward with this.
Happiness will never come to those who fail to appreciate what they already have.