RPO777 answered:
This defense would likely not even have occurred to Niobe historically speaking, as adultery in Roman conceptions dealt not with the “criminal” intent of the wife in terms of her faithfulness to her husband, but in the loss of dignity that the husband suffers as a result of the wife’s infidelity. Sexual purity was a posession held by the husband, even the rape of a wife would have been dealt with as a loss of dignitas on the part of the husband and a likely legal basis for divorce.
This is not directly on point, but it should be a good way to recalibrate how you think about sexual purity laws as they were conceived of in Roman times:
https://repository.law.umich.edu/cgi/viewcontent.cgi?article=1084&context=mjglThe paper deals with how Roman law dealt with the topic of rape, and does touch on issues of adultery.
This line is illustrative:
"The various choices of legal charges for rape punctuated the paramount importance of the Roman woman’s sexual integrity. "Rape could not be seen as invasion of a right to choose her own sexual partner so much as the destruction of her chief commodity in the exchange which accompanied marriage and which she was not equipped to negotiate., 63
The extreme value of a woman’s sexual integrity can be seen in the way raped women were treated by society and their families. Instead of being seen as victims, raped women were seen as sources of embarrassment to their husbands and fathers.6 With the loss of their virginity, unmarried women had little hope for a marriage, and married victims suffered shame and despair.65 The requirement of keeping their daughters and wives untainted for their reproductive capacity was of such utmost importance that some families tried to dispose of rape victims, for they could not be trusted with their primary function, legitimate reproduction.66 Adding to the facile rejection of the raped woman was the fact that the rapist was usually conceived of as a stranger who penetrated the family and the home from the outside.67
Thus the rape victim could be considered the weak point through which the stranger was able to invade the home, and as such, the family might eliminate her to hide evidence of the past and prevent possibility of future encroachments." (Nguyen 11)
Under Roman Republican Law, the pater familias was tasked with determining the punishment for the action, and permitted the killing of both the adulterer and the “unfaithful” wife.
Generally, Roman Law required intent to levy punishment. However, in cases of rape issues, the law was primarily concerned with maintaining the dignity and the rights of the husband or father, who was considered to be the primary injured party–not the woman.
In that legal context, I think it’s unlikely if a wife’s knowledge of her husband’s status of being alive or dead would have mattered in terms of the “injury” that she made to her husband’s dignity.