Ok, if it wasn't adultery, then what was it?
Your definition fits this case perfectly.
Unless you are trying to say that the wife committed adultery but not the boyfriend.
A weak attempt at definitional revisionism. According to the definition, anyone who engages in a voluntary act of sexual intercourse involving a married person and someone other than his or her spouse is an adulterer. This standard definition is completely independent of which party is married or whether they even know the other person is married.