Dad and mom could first remind their child of the Apostle Paul’s response when addressing a similar question, “Every man according as he purposeth in his heart, so let him give; not grudgingly, or of necessity: for God loveth a cheerful giver” (II Corinthians 9.7). The reasons (Paul, “purposes”) for giving to God’s work, His workers and to others is a private matter of the heart. If others cannot give joyfully, we might feel sorry for them, but we cannot allow their attitudes to affect ours.
God hates an unthankful attitude. Receiving an abundance of provision from God and then failing to respond with a desire to serve Him and an attitude of joy and gladness brings the judgment of God.
Moreover all these curses shall come upon thee, and shall pursue thee, and overtake thee, till thou be destroyed; because thou hearkenedst not unto the voice of the Lord thy God, to keep his commandments and his statutes which he commanded thee: And they shall be upon thee for a sign and for a wonder, and upon thy seed for ever. Because thou servedst not the Lord thy God with joyfulness, and with gladness of heart, for the abundance of all things (Deuteronomy 28.45-47).
If there be among you a poor man … thou shalt open thine hand wide unto him. Thou shalt surely give him, and thine hand shall not be grieved when thou givest unto him: because that for this thing the Lord thy God shall bless thee in all thy works (Deuteronomy 15.7-10).
Perhaps without making an indiscreet direct reference to the wealthy men in the church, the parents can help their teenager connect the passages to the recent conversation that was overheard.