In reality, all Christians should be living testimonies and witnesses to the saving power and grace of Jesus Christ. The Bible quotes Jesus at Acts 1:8 as saying, "you shall be MY witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth." Of the thousands of existing ancient manuscripts of the New Testament, not a single one contains the tetragrammaton, yet the "Bible" that the Jehovah's Witnesses use inserts the name "Jehovah" over 200 times in the Greek Scriptures.
My changed life is NOT the result of a man-made organization's power over me, scaring me straight through the threat of disfellowshipping or being destroyed at Armageddon. It IS the evidence of the awesome power of Jesus Christ to change lives through the indwelling Holy Spirit.
Have you ever considered that the depiction of lambs associated with the nativity could be symbolic? Jesus IS the Lamb of God who takes away the sin of the world! (John 1:29) Also, Jesus was revealed from Migdal Eder (the "tower of the flock"). This was not the tower for private flocks that pastured in the wilderness outside of Bethlehem, but specifically for the flocks born and pastured within a certain square area which were ultimately destined for the Temple sacrifices. (Mishnah, Baba K. 7.7; 80a) These particular flocks within this area were also out year round, unlike the ordinary flocks that were brought in from the wilderness in the harshest months. (Mishnah, Shek. 7.4: These flocks were out to pasture even during the rainiest month of February.)
For a discussion of the fact that Christmas is NOT of pagan origin and this idea that it made an easy transition for pagans converting to Christianity is actually a myth, see my answer here:
Edit: Perhaps you should ask your friend for the reasons American JW's refrain from celebrating Thanksgiving Day (idk if the UK has a similar holiday). In the O.T. God instituted two special annual festivals--the Feast of the First Fruits (Lev. 23:9-14) and the Feast of Tabernacles (Lev. 23:33-36) as days for the people to give thanks for the material blessings He had bestowed upon them. Thanksgiving Day in the U.S. can be traced, not to any pagan origin, but to the persistence of one Christian lady in particular who petitioned a number of Presidents in a row to set aside one day a year as a national holiday on which people should give thanks to God for His provision. Thanks largely to Sarah Hale's efforts, the U.S. became the first nation to establish a national holiday specifically for giving thanks to God.
In order for the Watchtower Society to be consistent with the reasons it gives for not observing Christmas, and in light of the clear instructions in the Bible to set aside special times to be thankful for the material blessings we receive from God, then why do JW's not observe Thanksgiving Day? I'll tell you why, because you won't hear it from your friend. As the Watchtower Society notes in it's "Bible Teach" booklet on page 159, "These occasions also give families opportunities to draw close together." The main reason the Watchtower Society is against observing holidays is because they are out to break up families, and I can tell you based on my years of experience in ministry to JW's, they have done a very good job of it. The Society does not want the Witnesses to associate with their family members who are not Witnesses, and particularly with any who are born-again Christians. Their holiday policy makes quick work of dividing them from non-Witness family.