I am not really the right person to answer, since I am more of a dominant, and totally a well endowed white man, not a hub who shares his wife. However I have had a lot of sex with wives, and mainly with their hubs present, and often watching. I can tell you a few things.
Do you want to watch? Is she ok with that? Here are some things to be sure of.
Be sure she is ok with the arrangement -- her going out with the guy, them staying at your house, you being in the house, or you watching.
Are you going to ask all sorts of questions about what they did and how she felt, when you are alone? Is she ok with that?
Make sure you get the right guy. Not only white and well endowed, but also basically respectful of you both.
If she wants you there, what do you want to do? Jerk off? Lick her clean? Suck him? How does she feel about that?
Make sure you do not distract her with your neediness. This is for her pleasure, after all. If she has to worry about you and your feelings, it may not be worth it.
I can't tell you what to expect to feel, but I have observed that hubs often feel a mixture of things -- humiliation, huge sexual excitement, pain at seeing your wife give herself to another man in the most intimate possible way. If you are not well endowed, maybe she will have sensations, even emotions, that you have never given her, and they may share these secret feelings (believe me this happens, I experience it a lot). Some hubs are both hurt by all this and love it more than anything, both at the same time. There are others who don't feel any of the hard stuff at all... but I have to say, those guys don't seem to experience the ecstasy of it either.