What about asking him if "legalism" is apostasy?
If he gives a half baked explanation, maybe ask him to define "legalism"?
*** g79 6/8 p. 28 Why the Emphasis on Christian Freedom? ***
...legalism... constitutes a denial of Christian faith
*** g79 6/8 pp. 27-28 Why the Emphasis on Christian Freedom? ***
Failing to appreciate this vital truth, certain Christianized Jews in the first century C.E. insisted that salvation could not be gained apart from the Law. Believing that acceptable moral conduct depended on strict adherence to the Law, they wanted to impose it on believing non-Jews. (Acts 15:2, 5) Such Christianized Jews lost sight of the fact that the Law in itself could not ensure fine conduct and that God’s spirit operating on those who exercised faith in Christ is a far stronger force for righteousness. (Gal. 5:16-18) The spirit of God produces love within the individual, and “love does not work evil to one’s neighbor.”—Rom. 13:10.
Persons who insisted on Law observance denied the value of all-essential faith and continued to look to works as the means for making themselves righteous. Their legalistic approach in matters of worship would have brought Christians back under an arrangement that only exposed individuals as being sinners and deserving of death.—Rom. 3:20; 6:23.
Hence, insistence on Law observance as a means for gaining salvation was really a returning to a slavery from which Christians had been set free on the basis of Jesus’ sacrifice.