Possible ending for this plot: the betrayal causes either the single mother or the neighbor to kill the other one, and commit suicide.
Who might, from my viewpoint, have the best expertise to handle this plot, again:
1. Lars von Trier (special aptitude, and in-depth exploitation, on such topics; will include a lot of full-frontal scenes)
2. Joel & Ethan Coen (to make the plot slightly farcical and less heavily-themed)
3. Darren Aronofsky (not bad; he can replicate something greater from Requiem for a Dream)
4. David Lynch (may be too surrealist and dark)
5. Atom Egoyan (sorry, again, you’re on bottom of the list; this plot might have a Rotten Tomatoes rating of mere 10%)