Is this kufr?

I told this person that saying nauzubillah merry Christmas is haram that person just laughed and still said it i told that person that the word itself is haram so she undertood after that now she says her intentions were not to do shirk and she is married does her marriage became void?

Congratulating disbelievers on religious occasions is illicit as it acknowledges their rites and worship (Zuhayli, Qadaya). It does not make faith void however as that is not intended.