Trust is important, but I think it goes a lot farther than that. I can have 100% trust and security with my partner, but if I care about him, I am still not going to want to watch another girl pleasing him. That's MY job. Some things are better left to fantasy, those situations are almost impossible to navigate if we are talking about a first time thing. The couples who pull it off only do so because both parties already have an existing detachment from the intimacy of sex (not uncommon for men, but pretty uncommon for women) and understanding of what their reaction will be.
Even if you don't touch the other girl, can you be sure one day that she won't get the little seed in her head, "I wonder if he was more turned on by watching her than he was by watching me?" Add that to this being a couple you know, and you see this woman at work, and it's going to fester. I guess this is where trust comes in, but it's impossible to know when you are in uncharted waters.
If you ever DID do this with her, I'd say you would have to figure out a way to do it with a couple or woman who you could sever ties completely with afterward. You would have to take steps that went above and beyond your normal communication routine to make sure this didn't put any cracks in the pavement.
what a mind fuck
I never thought about it from that perspective