A single judge of Kerala High Court held that the level or standard of prove for adultery is just preponderance of  probability under Sec 144 BNSS case (being a civil proceeding) but not that of Proof beyond reasonable doubt which may be applicable in a criminal case.

From Para 5,

5. Section 125 of Cr.P.C. (Section 144 of BNSS) states that a husband with sufficient means is liable to provide maintenance to his wife who cannot support herself. However, the wife’s right to claim maintenance is not absolute. Sub-Section (4) of Section 125 (Section 144(4) of BNSS) clearly specifies that a wife living in adultery is not entitled to claim maintenance. The dictum laid down in all the decisions cited by the learned counsel for the petitioner and referenced earlier is that a single instance of adulterous conduct is not enough to disqualify a wife from claiming maintenance; rather, there must be evidence of continuous adulterous behaviour. In brief, there should be proof that the wife is habitually engaging in an adulterous life with the partner to invoke the provisions of subsection (4) of Section 125 of Cr.P.C. This legal principle is well settled. The key issue, however, is that when a husband defends proceedings under Section 125 of Cr.P.C. (Section 144 of BNSS) by claiming that the wife is living in adultery, what level of proof is required to establish that the wife is indeed living in adultery?

From Para 6,

6. The right claimed by the wife under Section 125 of Cr.P.C. is a civil right. Maintenance proceedings under Section 125 of Cr.P.C. are also civil proceedings, although breach may lead to penal consequences. In criminal cases, the standard of proof is proof beyond a reasonable doubt, whereas in civil cases, the standard is based on the preponderance of probabilities. The concept of proof beyond a reasonable doubt applies only to criminal trials and cannot be used in civil disputes, including matrimonial and maintenance cases. When the husband alleges that the wife is living in adultery and thereby disqualified from claiming maintenance, he is not required to prove the adulterous act beyond a reasonable doubt, as in criminal prosecution under the now-repealed Section 497 of IPC. Instead, proof by preponderance of probabilities is sufficient. Adultery typically occurs in secrecy, making direct proof rare. Consequently, adultery can often be established through circumstantial evidence, provided the circumstances lead logically to that conclusion.

Index of Maintenance Judgments under Sec 144 BNSS is here.