The words “is in” are supplied. The sentence may also be translated, “the man who keeps instruction is a way to life.” This would seem to indicate that his wise counsel and good example make the wise man a guide, directing others in their pursuit of life. On the other hand, the translation of the KJV makes an equally true observation.
Erreth.
The form of this verb in the Hebrew is causative, suggesting that the one who refuses to heed instruction and reproof leads others astray.