What is the consequence for a wife who placed a Qur’an between her thighs to swear she would not betray her husband, then denied this oath, knowing that her husband found her betraying him?

The Mushaf (copy of the Quran) should not be placed between the thighs. And whoever swears by the Mushaf is like one who swears by Allah, because the Quran is the Word of Allah and an attribute of His attributes. If the one who swore by the Mushaf breaks their oath, they are liable for one expiation for an oath (kaffarat yamin). It is recommended to repeat the expiation according to the number of verses for those who are able. So, if you utter an oath, such as saying, "By the right of this Mushaf, I will not betray my husband," and then you break it, then one expiation for an oath is due if the oath is repeated for the same thing. And she must offer sincere repentance for betraying her husband.