Is it sufficient to offer only advice to someone who disobeys God secretly and whose disobedience does not harm others, or is something else required to help him?

Enjoining the good and forbidding the evil is a communal obligation (fard kifayah), and it may become an individual obligation if no one else knows about the evil or if only that person can remove it. Therefore, the aforementioned person should be advised with wisdom and gentleness, clarifying the seriousness of sins and the punishment for those who commit them. If you volunteer to help him, you will be rewarded.