Witness defenders have a real answer here. Say so before your friend does.
Witnesses may not keep birthdays or holidays. Keeping one, and not backing down, can bring a hearing.
The birthday case runs like this. The Bible records only two such parties.
One is Pharaoh's (Genesis 40:20-22). One is Herod's (Matthew 14:6-10).
Both men were pagan kings. Both parties ended with a death.
So God's servants do not keep them.
It argues from silence. Then it blames a custom for where it came from.
The deaths were down to tyrants, not to cake. The Bible also shows Job's children feasting each one on his own day, with no rebuke (Job 1:4).
And no verse forbids it. That same test would rule out wedding rings and the names of the weekdays.
The Watchtower allows both. So the test is not applied evenly.
The real case is better than the caricature. No early Christian kept birthdays.
Origen wrote that only sinners, such as Pharaoh and Herod, mark the day of their birth. So the Witness reading has early-church backing.
Cakes, candles and wishes do come down from pagan charms, per standard reference works. Nobody is harmed by sitting one out.
Christmas rests on a date and customs that are not Christian, and on a nativity timeline many non-Witness scholars doubt. Standing apart honours a conscience aimed at pure worship (2 Corinthians 6:17).
Not in standing apart. In being made to.
Paul leaves days to each person. Romans 14:5-6 says it plainly: "let each one be fully convinced in his own mind." One man rates one day above another.
Another rates every day alike. See also Colossians 2:16.
Watchtower policy treats a birthday as proof that you have left, with family shunning downstream. Note also that the movement kept Christmas until 1928, and birthdays until about 1951.
Genuinely arguable, both ways. Sitting these out is a fair use of liberty, and it has early-church backing.
Mocking it plays badly. Aim only at the force, never at the quiet conscience.
"I have no problem with you skipping birthdays. My question is why Romans 14:5 does not make it your own call."

No early Christian kept birthdays, and Origen said only sinners mark the day.
The steelman is stronger than the caricature. No early Christian kept birthdays. Origen wrote that only sinners mark the day of their birth, and he named Pharaoh and Herod. So the Witness reading has early-church backing.
Cakes, candles and wishes do come down from pagan charms, per standard reference works. And nobody is harmed by sitting one out. Christmas rests on a date and customs that are not Christian. It also rests on a nativity timeline many non-Witness scholars doubt. Standing apart honours a conscience aimed at pure worship (2 Corinthians 6:17).
A fair fight. Aim at the force behind the rule, not the quiet conscience.
Graded conservatively. Standing apart is a fair use of liberty. It has real early-church backing, and mocking it plays badly.
What is not contested is the enforcement. Romans 14:5 leaves days to each person. Watchtower policy makes a birthday proof that you have left, with family shunning downstream.
Aim the case at the force, not at the quiet conscience. Severity low.