My personal vote is that your wife should not go to the other guy's place if you and she are in the midst of an argument (unless she had already made a prior commitment with him to see him at that time). First of all because it only seems fair, after all you have no additional woman to run off to and seek comfort, secondly because you have always worked things out in the past without needing a third party involved. If she needs to take a drive to clear her head that's fine, but visiting the other guy at that time would be inappropriate in my mind.
Maybe your feelings will change eventually, and you will start to feel okay about her seeking out the other guy for comfort (and maybe sex). If you do start to feel okay about that, you can let her know your feelings have changed and re-negotiate that rule.
This is just my vote of course, and it is up to you and your wife what agreement you want to come to. It sounds to me like you are okay about her (when you and she are arguing) seeking out the other guy for comfort, but not for sex. That seems reasonable enough to me, if she's willing to stick to that rule.
An argument is a very delicate time. It is not a time to engage in behaviors that will make one of the people feel much worse. I think the argument should be resolved before she goes to see the other guy. Unless, like I said, she'd already made a promise to see him at that particular time and it just turned out to be a bad time. But even then, I am wondering in my mind if she shouldn't call the guy and say, "We are in the midst of an argument and we need to get it resolved, can I reschedule my time with you?" I'm not sure whether it should be handled that way, but I'm not ruling it out.
Hopefully you and she can work out an agreement that you can both live with, and stick to the agreement. It is good that you are thinking and talking about this contingency ahead of time.
Love means never having to say, "Put down that meat cleaver!"