I’m not sure if she was that motivated by money. She had loads herself, of course, which helps, but she readily gave those loans to Simon’s family.
I think she had very rigid ideas about relationships and loyalty. Only a very immature person wouldn’t appreciate that, after a marriage separation, her (ex) in-laws would be most likely supporting their own son. Whereas Erin seems to feel betrayed.
Her life seemed quite small - limited real life friends, solitary hobbies (Lego) didn’t work, a shift in schools would have limited friendships with other school parents, so not much to balance out or give perspective to her grudges and self-pity.