[HEZ-704] I’ve been tending to my son’s sexual needs 365 days a year, morning to night, because he never leaves his junk-filled room on the second floor. Kawakami Yuu

The very first thing the woman says to the mother is “hajimemashite” which is similar to a phrase like “nice to meet you”. She then hands the mother a gift, common practice for new neighbors coming to meet for the first time. The mother replies “sumimasen, waza waza go aisatsu ni”, which is basically thank you for going thru the trouble to come introduce yourself.
That’s how we know it’s a neighbor and not any of the other things you said.
At the end when they are caught, the neighbor says “okane kashite kurenai?” which is basically wont you pay up? So they move out.