I often talk about how I struggle to understand how casual sex is so much less threatening than feelings, but what your wife feels, is what she feels.
Understand...you cannot control your feelings, necessarily, only the words and the actions that you choose to take, regardless of how you feel.
This is a very important concept, because it applies to your feelings for your friend, AND it applies to how your wife feels about you having feelings for your friend. You feel what you feel...she feels what she feels. And your friend feels whatever she feels, whether she is getting emotional for you, or whether her feelings were not so deep and it's more important to her, to avoid harming your marriage. Her feelings are also valid and should be taken into consideration.
One point that I feel is important... You mentioned "commitment." Feelings do not have to mean commitment. This is another idea that is part of the "monogamy hangover"...or the bundle of concepts that we are taught about relationships. That there is this clear process of how things go, that emotional bonding means that you have to DO SOMETHING. It is possible to feel things for someone, but have it not alter the parameters of how you do business in the relationship. It can simply mean, "I truly enjoy this person and want to share time and life with them" without meaning that they need to become part of your household or family or anything permanent.
Take it apart. Look at every piece as separate. Maybe that helps.
Ultimately when people are not on the same page about what is allowable in the relationship, whether it is open or closed, it comes down to negotiation. What do you want, what do you need? What is your hard line, and what is most important to you? Where are the areas of flexibility?
I usually advise people not to legislate emotions. Making rules that your partner is not allowed to develop feelings for another, is often doomed to fail ultimately. I believe that any time that sex is involved, there is always the possibility that feelings may spring up whether we intend them or not. To allow a partner to explore sexually but tell them that if they develop emotional bonds, they must cease contact with the subject of their affections, is in my personal opinion, cruel. And you'd be better off not opening at all, if that's how it is, or at least that's how I would see it.
If a partner told me I could have casual extra-relationship sex, but not allowed feelings, I would NOT have the casual sex. I would not expose myself to that hurt, nor expose hypothetical third parties to it either.
But again...that doesn't mean that one must make "commitments" exactly. Sometimes those feelings will run their course. I would want to be able to experience them, and go forward with them, and let them happen and evolve. Love does not mean leaving someone for someone, or needing to live with someone, or anything like that, after all.
Best of luck with your negotiations.