بِوَجْهٍ طَلْقٍ means a pleasant, open expression. Listening attentively, speaking gently, and avoiding needless harshness give everyday opportunities for good conduct. It is not a rule that a distressed person must force a smile at every moment.
Example 1: A shopkeeper treats a visitor courteously even if no purchase is made.
Example 2: A teacher corrects a mistaken student calmly instead of humiliating them.
طَلْقٍ is an adjective describing وَجْهٍ. Both carry the genitive ending in this prepositional expression.