Its a single mom fucking with her sons best friend. So its not really cheating and the ntr tag only fits because the definition says "[...]to invoke jealousy[...]" but then again its her son and not her lover.
If you'd ask me this maybe ntr but not cheating, and the ntr is weird like in some of the "Akatsuki Souken" works where the son feels betrayed/jealous because her mother has a sex life? weird