She likes her friend's mom... How is that ntr lmao. She literally hasn't shown any concrete signs of interest in him either so it's not like she's not even leading him on wtf. He understandably thinks she likes him but that's mostly on him for not just straight up confirming.
NTR is when they are in relationship (her and her classmate) before she is "stolen" by mom. All that this poor guy has for now is wet dreams. So, technically it won't be NTR. Though the guy would most probably feel the same