"If" being the objective word...if that was to ever happen I'd prefer a complete stranger.
My reasoning is simple. If the other guy was to make her cum/squirt hard, she might get "hooked" on that cock if it belonged to someone close. Then I might have to deal with the aftermath of my wife longing for more of it. If it's someone she has zero chances of staying in contact with, then it's just a nice memory.
I'm pragmatic about these things lol!