Though I agree that the married lady can be charged with adultery, I guess the question is, statistically speaking, how many ex-husbands actually sue their ex-wives for it? This particular ex-husband is supposedly in the US and is more likely than not, already in a relationship there. The burden of proof will be on the person filing the case against them and this can only be the offended spouse aka ex-husband who is in the US. Let's remember that the visa officers who will be in charge of reviewing the application will be from Manila who are very much aware of Philippine laws.
It doesn't matter how often it happens. A person can be charged with adultery for living with someone other than their legal spouse. IRCC does not and cannot expect a person to risk criminal charges in order to become common-law. Situations such as this are exactly why conjugal was created.