@ author .
The Courts have held that the evidence needed is evidence of inclination and opportunity not graphic evidence of private activity in the bedroom. Inclination could be established by statements of affection in correspondence. Evidence of opportunity could be established through evidence that the two people lived together, traveled together or stayed in a hotel room together or were alone together at a time and place and for a duration which might be called "compromising".
In case of adultery very often direct proof of the act can not be made out as it is a pure privacy acts betwen two individuals, as such circumstancial evidence can be put forth to prove the offence . without tampering; voice recordings, original media materials in which different voices and SMS messages captured should be submitted for forensic examination along with voice samples of accused and witnesses as per Evidence Act
Courts has upheld instances of adultery of wife in more or less following cases which bear relevance to your alleged facts;
(a) Wife had been absenting herself from her house for some times and seen in the company of a stranger to the family of her husband without reasonable explanation or any explanation.
(b) Evidence on post-suit adultery is admissible to prove and explain other evidence given in the case and to show the character and quality of the previous acts of wife.
(c) Admission of adultery by wife through voice recording.