I personally enjoy confronting them. They usually leave in tears.
Anyhoo, ask them these simple questions. Was Yoshke the Messiah? They will say yes. Ask if the Messiah needs to be from King David. they will say yes. Ask them who Yoshke’s dad is. They will say G-d. Ask if G-d is a descendant of King David. See, since Xtianity is a mixture of Judaism and paganism there are many contradictions. Joseph, Mary’s husband is ben David according to the NT, but also according to the NT he was NOT Yoshke’s father, G-d is. They will not be a able to reconcile this. They will declare that Mary is from David. You can show that it has to be ben David not Bat David.