The affix vun comes in the sense of 'versed therein or skilled therein' after the word pathin being in the Locative -7th case in construction.,
Thus पथि कुशलः = पथकः (पथिन् + वुन् = पथ् + वुन् 6.4.144 = पथ् + अक् (VII. I. 1.).,
तत्र कुशलः पथः
Adhyāya 5 · Pāda 2 · Rule 63