That doesn't explain how it's NTR.
Are she and the MC actually dating and in a relationship when this happens, is he in move with her when this happens?
Because for it to be NTR there needs to be some kind of relationship or feelings between them first, otherwise it's just her having sex with someone else.
And if it's the scene and character I'm thinking of it literally gives you the option to get involved and make it a threesome, and when I got it the characters were at best acquaintances with benefits.