We tell a person to commit a minor sin in order to save his friend from a more severe sin.

אומרים לאדם שיחטא חטא קל בשביל להציל חבירו מחטא חמור

The concept you're referring to is a topic of discussion in Jewish law and ethics, and it addresses the question of whether one is allowed to commit a minor sin (חטא קל) in order to prevent someone else from committing a more severe sin (חטא חמור).

The general ruling, according to most halachic authorities, is that one is not permitted to transgress even a minor prohibition to prevent another person from committing a more serious sin. This principle is based on the idea that each person is responsible for their own actions, and one should not compromise their own spiritual integrity, even with the intention of helping someone else avoid a worse transgression.

A well-known source discussing this is in the Gemara (Shabbat 4a), where it is explained that one does not commit a sin, even a minor one, to save another from a greater sin. This principle underscores the importance of each individual's accountability in their relationship with Hashem.