Dear XXXXX, Merry Christmas and Happy New Year! Merry Christmas Happy Holidays Best wishes, Your friend, YYYYYY Sincerely, YYYYY

oh , I'm not sure , I believe the important thing is mutual support and understand , so both are friend .

Salutation: Happy Holidays, [name]! -OR- Merry Christmas, [name]!
Closing: From, [your name]

If you don't the person's religion for sure, "Happy Holidays" is the safest greeting to use.