No, he is not. For one thing, he's playing with fire; even if he hasn't as yet entered into a sexual relationship, he very likely will in the future. He may say he'll never go down that road - but he's already traveling it, whether he realizes it or not. The Bible says, "Avoid it, do not travel on it; turn from it and go on your way" (Proverbs 4:15).
But what he is doing is wrong because he's already breaking his marriage vow. When my wife and I were married, we promised to forsake all others and to be faithful to each other until we were parted by death. Undoubtedly, this man and his wife made a similar vow - a vow not only to each other, but to God. But now he's broken that vow, for he is giving his love to another woman. Instead of forsaking all others, he is now including another woman in his life.
The tragedy is that eventually his duplicity will catch up with him. Where will this relationship lead in six months, or a year? No matter what happens, the results won't be happy. The Bible's warning is clear: "You may be sure that your sin will find you out" (Numbers 32:23).
Pray for him, that he will see the error of his ways and turn from them. Pray most of all that he will yield his life - and his marriage - to Jesus Christ. Only then will he find true peace.