Exodus 20:14
Thou shalt not commit adultery.

What is adultery? Lev 20:10 defines it:

Leviticus 20:10 And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbour's wife, the adulterer and the adulteress shall surely be put to death. [See also, Proverbs 6:32-34, Jeremiah 29:23, Ezekiel 16:32]

Adultery is defined simply as a married woman lying with a man other than her husband. They are both committing adultery.

Note: Don't confuse fornication with adultery. Fornication is an act with a woman who is not one of your wives or an animal. Fornication is also a sin.