All i can say is be prepared for her to go ape ...... horny with the guy and fuck him like she never has with you.
You'll probably feel hurt and jealous.
What kind of rules have you discussed? Will they kiss? They gonna use rubbers? She gonna let him cum in her mouth?
You only going to watch? Where will it happen...home? Motel?
Will she have sex with you after?
Let us know kinda what you two have talked about.......is he older, younger, black??
We get into mfm fun with a guy we know, it's not cuck at all. We've been doing it with him about 4 years, many, many times. I still get weird jealous feelings wathing them have sex. Some times I think she likes it way too much with him.