That's a pretty specific set of circumstances. Why would we care about what someone does when we can't see them and don't have anything to do with them? How would we even know what they do? He said she said? It doesn't matter whether they continue to be part of the story or not... If the protagonist has rejected any possibility of a romantic relationship with them... they do whatever. It's not ntr and anyone who says it is... is an idiot and should be ignored. Hell, now that I think about it... let the definitions speak...
Netorare is a genre of anime, manga, or video game focused on infidelity. A genre of cuckoldry pornography where a protagonist's love interest or close associate is corrupted and taken away by an antagonist.
Netori is a counterpart tag to Netorare, however instead of playing as someone who has his significant lover stolen, you play as the one doing the stealing.
And just for good measure...
Infidelity is the breaking of a promise to remain faithful to a romantic partner.
Someone who has been rejected is NOT a lover, love interest, partner or a significant anything. Therefor it is not possible to meet the requirements of any ntr definition.