So the friend looks a whole lot like the wife next door. He doesn't, however, look at all like the rest of his family. The wife seems estranged from her marriage and while the friend isn't outright abused in the usual sense of the word, he is all but ignored and abused (made use of) as a drone in the house as all he is to them. A kid that should be seen and not heard and all that. So, resentment, but not outright hostility or mistreatment without plausible reason (like that one expected phrase for an excuse to get to chide him).
If the genders were different, I'd bet on the kid being the neighboring wife's kid from a tryst as an open, marriage straining secret, but the logistics of the cuckoo bird being a woman here are pretty damn hard. Some sort of cheating or other fuckery is definitely not off the table: Those two households are a bit more closely related than they'd say aloud, I'd bet.
And the kid isn't fully theirs, that's for sure.