That's a funny inception. I wouldn't say that completely validates, or dissolves, the potential of this being NTR since it's still the guy in question, just no on a technicality. She was willing to indulge him after all which says she was convinced of her love towards him for being the guy. But it's technically not the same guy exactly so legally speaking it wouldn't fly. Hmm, judge is gonna have to be out on this verdict me thinks. All the same despite loving the protagonist, she still loved the guy she married as well despite the oddity she didn't know about, making this more of a swinger or polyamorous occasion that turned sour once the protag took it the wrong way. Regardless of that, the fact she was willing to share her love despite the infidelity, and there are multiple if you change landlord to something else, leads me to believe she's too loose of a women to believe in concepts of undivided love. There's a bit of Netori, 'cheating', and just in general depravity. NTR kinda requires unconditional love to actually exist as a concept, otherwise you can replace it with the myriad of other subjects such as the ones I mentioned here.