As you probably know, Taoism (pronounced "dow-ism") is an ancient Chinese religious philosophy commonly attributed to Lao Tzu, the "ancient philosopher." The teachings of Taoism are found in the Tao Teh Ching, an epic poem of 81 stanzas. If you were going to engage a Taoist, this would be the place to start. Get the Tao Teh Ching and read it carefully. What you will find are surprising parallels to Christianity. There are vast differences, sure, but the similarities will give you a place to start. Take a look at some of these parallels.
The Tao, the uncreated cause, is transcendent: "In the beginning was the Tao. All things issue from it; all things return to it" (v. 52). "In the beginning was the Word and the Word was with God, and the Word was God. . . . All things were made by him; and without him was not any thing made that was made (John 1:1–3).
The Tao is immanent in all things: "It flows through all things, inside and outside, and returns to the origin of all things" (v. 25). "In him [God] we live and move and have our being" (Acts 17:28).
The Tao is engaged in creation, which it cares for: "The Tao gives birth to all beings, nourishes them, maintains them, cares for them, comforts them, protects them . . ." (v. 51). "I [Yahweh] have nourished and brought up children . . ." (Isa. 1:2); ". . . and upholding all things by the word of his power . . ." (Heb. 1:3.); ". . . casting all your care upon him; for he cares for you." (1 Pt 5:7); ". . . for the Lord has comforted his people" (Is 49:13).
The Tao warns against judging: "If you close your mind in judgments and traffic with desires, your heart will be troubled. If you keep your mind from judging and aren’t led by the senses, your heart will find peace" (v. 52). "Judge not, that you be not judged. For with what judgment you judge, you shall be judged, and with what measure you measure out, it shall be measured to you again" (Mt 7:1–2).
The Tao teaches universal love: "Love the world as your self; then you can care for all things" (v. 13).
"You shall love your neighbor as yourself." (Mt 22:39). "But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you and persecute you" (Mt 5:44).
The Tao teaches a way of gentleness to overcome force: "The gentlest thing in the world overcomes the hardest thing in the world" (v. 43). "Go your ways: behold, I send you forth as lambs among wolves" (Lk 10:3).
The Tao teaches the paradox of dying to self: "If you want to be reborn, let yourself die" (v. 22). "For whosoever will save his life shall lose it, and whosoever will lose his life for my sake shall find it" (Mt 16:25).
The Tao teaches the paradox of giving up possessions: "If you want to be given everything, give everything up" (v. 22). "And every one that has forsaken houses, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my name’s sake, shall receive a hundredfold and shall inherit everlasting life" (Mt 19:29).
The Tao teaches the transitory nature of riches: "Chase after money and security and your heart will never unclench. Care about people’s approval and you will be their prisoner" (v. 9). "Lay not up for yourselves treasures upon earth, where moth and rust corrupt and where thieves break through and steal" (Mt 6:19).
Stressing these and other parallels may soften your friend’s heart and allow him to hear the gospel anew. But keep in mind that there are two huge differences between the religions. Despite the similar language, the Tao and Yahweh are vastly dissimilar. Taoism does not admit a Creator who is conscious and personal or who strives for us. Neither does Taoism admit original sin, though it recognizes destructive behavior. In Taoism there is no need for salvation, only enlightenment.
I suspect that your friend is attracted to the mystical and philosophical aspects of Taoism. It could be that his own Christian tradition is devoid of these. Perhaps you could whet his appetite with the writings of some of our Catholic mystics. I would recommend the work of Meister Eckhart whose work develops themes that are congruent with Taoism but also thoroughly Christian.
One more thing: Remember that conversion is not accomplished by us but by God. Pray for your friend and bring his case before the Lord who "desires all men to be saved and to come to the knowledge of truth" (1 Tm 2:4).