In the Philippine legal system, marital infidelity is not merely a private grievance but a criminal offense. When a spouse engages in an extramarital affair, the legal system provides specific avenues for the offended spouse to seek redress, not only against the erring partner but also against the "third party" involved.

Under the Revised Penal Code (RPC) and related jurisprudence, the legal grounds for filing a case against a third party vary significantly depending on the gender of the erring spouse.

1. The Criminal Charge of Adultery

Under Article 333 of the Revised Penal Code, Adultery is committed by any married woman who shall have sexual intercourse with a man not her husband, and by the man who has carnal knowledge of her knowing her to be married, even if the marriage be subsequently declared void.

Grounds for Liability of the Third Party (The Paramour):

Knowledge of Marriage: The man must have known that the woman was married at the time of the sexual act.
Sexual Intercourse: The essence of the crime is the physical act of sexual intercourse. Mere intimacy, suggestive messages, or "emotional affairs" do not constitute adultery under the RPC.
Participation: The third party is charged as a principal in the same case as the wife.

2. The Criminal Charge of Concubinage

Under Article 334 of the Revised Penal Code, Concubinage is committed by any husband who shall:

Keep a mistress in the conjugal dwelling;
Have sexual intercourse, under scandalous circumstances, with a woman who is not his wife; or
Cohabit with her in any other place.

Grounds for Liability of the Third Party (The Concubine):

Knowledge of Marriage: Like adultery, the woman must know the man is married.
Specific Conditions: Unlike adultery, mere sexual intercourse is not enough to convict a husband and his mistress. One of the three conditions above must be proven.
Penalty Distinction: While the husband faces imprisonment, the concubine is only penalized with destierro (banishment or a prohibition from entering a specific radius of the couple's residence).

3. Psychological Violence under R.A. 9262

The Anti-Violence Against Women and Their Children Act of 2004 (R.A. 9262) provides a modern and often more potent ground for filing cases involving third parties.

Psychological Violence: Infidelity is recognized as a form of psychological violence because it causes mental or emotional suffering to the wife.
The Third Party’s Role: While R.A. 9262 is primarily directed at the husband, the Supreme Court has clarified in various rulings (e.g., XXX vs. State) that a third party can be held liable as a conspirator.
Grounds: If the third party willfully participates in the infidelity, knowing it causes emotional anguish to the legal wife, they may be charged as an accomplice or co-conspirator to Section 5(i) of R.A. 9262.

4. Civil Liability and Damages

Aside from criminal prosecution, the offended spouse can file for civil damages against the third party under the Civil Code of the Philippines.

Article 26

"Every person shall respect the dignity, personality, privacy and peace of mind of his neighbors and other persons." Specifically, "meddling with or disturbing the family relations of another" is an actionable tort.

Article 2219

This allows for the recovery of moral damages in cases of adultery or concubinage. The offended spouse can sue the third party for:

Mental anguish
Fright and serious anxiety
Besmirched reputation
Moral shock

5. Alienation of Affections

While not a standalone criminal statute in the Philippines, the concept of Alienation of Affections is often integrated into civil suits. It involves a third party's malicious interference in a marriage, resulting in the loss of love, companionship, and consortium between the spouses.

Procedural Requirements and Limitations

The "Pardon" Rule

Under Article 344 of the RPC, the offended party cannot institute a criminal prosecution if they have consented to the offense or pardoned the offenders (both the spouse and the third party). A pardon for the spouse automatically extends to the third party.

The "All or Nothing" Rule

In adultery and concubinage, the offended spouse must include both the erring spouse and the third party in the criminal complaint. One cannot be prosecuted without the other if both are alive.

Quantum of Evidence

Criminal Cases: Requires "Proof Beyond Reasonable Doubt." This is notoriously difficult in infidelity cases as direct evidence of the sexual act is rarely available; circumstantial evidence (e.g., shared hotel rooms, public admissions, births of children) must be overwhelming.
Civil/Administrative Cases: Requires "Preponderance of Evidence" or "Substantial Evidence," which is a lower threshold than criminal cases.

Summary Table

Legal Ground	Governing Law	Primary Requirement for Third Party
Adultery	Art. 333, RPC	Sexual intercourse + knowledge of marriage.
Concubinage	Art. 334, RPC	Cohabitation, scandal, or keeping in the home.
Psychological Violence	R.A. 9262	Conspiracy to cause emotional anguish to the wife.
Civil Damages	Art. 26/2219, Civil Code	Proof of "meddling" or "disturbing" family peace.
Administrative Case	Civil Service Rules	Applicable if the third party is a government employee (immorality).