Yes, indeed, a person can be responsible for another’s sin. In this particular case, whether or not the person who wants a divorce is responsible for the spouse’s sexual sins outside of marriage depends on whether or not there exists serious, just reason for the divorce. If there is serious, just reason for the divorce, then the innocent spouse would not be at fault for the sins the other spouse might commit outside of marriage. But if one spouse simply wants out of the marriage and has no objectively serious and just grounds for the divorce, then he can indeed be responsible for the sins the abandoned spouse might commit due to the dissolution of common life. This does not mean that the abandoned spouse has no responsibility for those sins; it only means that the spouse who unjustly abandons him or her may share in the culpability for those sins.
