WANDERER: Broken Bed is not about NTR
I respect your opinion on multiple partners, but that scene shouldn't be specifically labeled as NTR. It currently plays out as a spontaneous consensual threesome. There is no dialogue of betrayal, a comparison between lovers, or anything unwanted by the unknown girl or her friend that would relate it to NTR.
We can suggest that the developer add "(threesome)" or "(join)" to the end of the choice that causes that scene. Then it can be clear and avoidable.