In giving advice seek to help, not to please, your friend.

This sentence, with slight variations, is usually attributed to the 5th century BC Athenian lawgiver, Solon. It has its roots in collections of sayings attributed to Solon in antiquity. There is some variation in the Greek words attributed to Solon in these collections. Some sources follow:

Anonymous, Septem sapientium dicta, ed. Wölfflin, Sitz.-Ber. Bayer. Akad., 1886, p. 287 sqq.

απασι συμβουλευε μη τα προσφιλη, αλλα τα πρεποντα

Diogenes Laertius, Vitae philosophorum, I, 60

συμβουλευε μη τα ηδιστα , αλλα τα αριστα

Demetrius of Phaleron, Septem sapientium dicta, fragment 114 Wehlri, ap. Stob. I (π. αρετης) 172 β', III p. 114 Hense

συμβουλευε μη τα ηδιστα , αλλα τα βελτιστα

The translation of these last versions, "Don't give the most pleasing advice, but the best," is a bit simpler than the version I was originally asked about, "In giving advice seek to help, not to please, your friend."