This is a tricky issue, principally, because California has abolished what is known as "heart balm" lawsuits (e.g., "intentional alienation of affections") Cal. Civil Code Sec. 43.5. Questions:
1. Has the employer (i.e., a manager of your wife or the coworker) been aware of this relationship?
2. Does the coworker have any authority to directly and adversely affect your wife's employment (fire, demote, reduce pay), or the ability to influence a manager with such authority?
3. Is there any extrinsic evidence of this relationship (third-party witnesses, email, texts, voicemails, etc.)?
4. Did the last occurrence of intimacy occur within the last 12 months?
Thanks in advance for your assistance.