Bad seeds / Mary Monroe.

Enhanced descriptions from Syndetics:

The daughter of a hard-working restaurant owner, Louise Brooks always sees the best in people-and in helping them no matter how difficult her own life gets. She's lived through tragic loss and running the family business on her own, even in the face of a failed marriage. So, she's delighted when she finds a best friend in Della Thornton, a woman struggling with bad breaks and unlucky romances. When Louise's father and her prosperous second husband suddenly pass away, Louise takes Della in and makes her the restaurant's right-hand woman as it grows more successful than ever... Louise is now convinced lasting love is not in the cards for her-until she runs into handsome Malcolm Purdy. He's everything she could want-outgoing, charming, and attentive. Soon they become engaged. And although Louise is dismayed that Della and Malcolm hate each other from the start, she does her best to keep the peace between the two people she cares about and trusts the most... But a chance encounter from the past shows Louise that neither Della nor Malcolm is quite who they say they are-and their deceit runs deeper and deadlier than she imagined. With her illusions in ruins and a newfound desire for vengeance, Louise slowly entraps the pair through lies, greed-and lust. But how far will she go to get justice? And will her final shocking move cost her more than she's willing to lose?