First off, Welcome to the board. I would suggest doing a lot of reading here.
Short and simple; if she is married, and they do not swing, it is cheating. Not trying to be uptight here, but that is quite frowned upon by those in the lifestyle. Consentual by all parties is swinging, doing it on the sly from her hubby is cheating
You might be able to broach the subject from a swinging direction.... could be fun facelick
Just my humble opinion.
Twofers