charge of adultery
The way adultery as a offence has been drafted, it is considered to be an offence against the husband of the lady who is involved in sexual relationship. So if the husband consents or does not object and if this can be proven in the court then the offence of adultery is difficult to be established.
Section 497 in The Indian Penal Code
497. Adultery.—Whoever has sexual intercourse with a person who is and whom he knows or has reason to believe to be the wife of another man, without the consent or connivance of that man, such sexual intercourse not amounting to the offence of rape, is guilty of the offence of adultery, and shall be punished with imprisonment of either description for a term which may extend to five years, or with fine, or with both. In such case the wife shall not be punishable as an abettor.
since your query is lil complicated. first you need to understand the law. the law says on charge of adultery it is the person who is complaining should have the strong evidenecs/ proof about it that his wife is in adultery. but here if the husbnd has never objected then dere is no case. has he has consented.
Moreover your friend should keep some evidenecs of the consent for the future purpose if hge ever falsely get booked for the same offence.