the childhood friend boy isn't really mc, at least from what was shown so far. she's not interested in him, he's just her first meal, and completely by accident at that, nothing more. so there being others is not ntr.
however if the next chapters ramp up from the no-actual-sex start to full on hentai it does become "ntr" in a completely different sense - not betraying a character, but betraying the style it started as, and betraying her declared intention to resist becoming a succubus (which she kinda already did by accepting this sus training task anyway). i'd probably hate that more than actual ntr which i've already learned to ignore.