Begin by asking what he thinks kneeling in prayer proves. That the Catholic religion is wrong? What if a Catholic never knelt in prayer or at Mass--would that somehow vindicate Catholicism?
If that line of reasoning doesn't show him how silly his quibble is, then try a few Bible verses.
- After withdrawing about a stone's throw from them and kneeling, he [Jesus] prayed (Lk 22:41).
- Peter sent them all out and knelt down and prayed (Acts 9:40).
- When he had finished speaking he [Paul] knelt down and prayed with them all (Acts 20:36).
- All of them, women and children included, escorted us out of the city, and after kneeling on the beach to pray, we bade farewell to one another (Acts 21:5).
Jesus, Peter, and Paul all lived "in Bible times," and they knelt when they prayed. Were they "unbiblical"?