I think the point of a "cool down" period mid-argument is to re-focus your thoughts without the spinning head and escalating emotions so you can approach the topic with a fresh view. It takes the confrontation out of your face so you can think clearly and stop accusing.
My husband is like your wife in that when something is bothering him, he likes to drive around and mull it over. Driving around gives him a chance to think about it by doing something basically mindless.
I don't see how having sex with someone else during that cool down period would help her mull things over. That's pure distraction.
It's not like her current method is to go out with friends and play cards or go to the bar, and now it's just a boyfriend instead of friends and sex instead of cards. Going to a boyfriend for sex instead of going out by herself for thinking time is completely different behaviour with accordingly different consequences.
For example, Boyfriend looks pretty darn good when the freshest memory of Husband is the heated argument. Repeat that enough times, you've got a recipe for Grass is Greener.
At the end of the day, you can't control her behaviour. If she's going to go have sex with her boyfriend to supposedly cool off, you can't really stop her. But I believe you would be justified in being bothered by that, and I see nothing wrong with letting her know that you'd prefer it if she stuck with the tried & true methods.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker