Yes. The w/gf could have sex with another woman. Thus she cheated on him, she cucked him.
The dilemma in this FFM scenario is that most men are turned by thoughts of and watching lesbian sex. The challenge is to convert the man's (the cuck's) excitement into humiliation. The cuck/man has to watch, but be denied enjoyment, he can't jack-off and he should probably be cock-caged, and the w/gf has to withhold sex from him all the time, completely. Importantly, the two women must not fall in love or be romantic. They're just sex machines getting each other off and showing the cuck what he cannot do. If the two women are in love, the man becomes irrelevant. What they are doing is for them, not to him. The man should walk away, get a divorce.
I watched my college girlfriend get fucked by another girl with a strap on. It was incredible to see her react to her friend's fucking so much more than being with me. That was even more humiliating (and hot) than to be cucked by another man