It is adultery (zina) if a married man has sexual intercourse with a woman other than his wife. Adultery is one of the great sins. Not only adultery but also approaching it is forbidden:
“And do not approach unlawful sexual intercourse. Indeed, it is an outrageous act and an evil way.” (Al-Isra/ The Night Journey 17:32)
The man had a great sin but it does not harm the marriage. If he repents for this evil act asking for forgiveness from Allah and his wife also accepts to continue their marriage, they can do so with no necessity for a new one.