LOL! You know all of us horny old fuckers will say "yes"!
Seriously, if they are at a point where you are worried about her falling for him, she needs to to do it to get it out of her system. Otherwise, it may create friction in the long term. Dangerous game, yes; but more dangerous to not let her go.
BTW, my wife used to take a private vacation once a year, staying with a married couple (the guy was an old class mate). Her first time I encouraged her to go, enjoy herself,a and do things one might do when away from home. And she did, with each successive year becoming kinkier, culminating in some hot 3 way sex where she ate pussy for the first time!
I loved it and recommend it for all married couples! I am confident you would too.