lol, I didn't even notice it was old until you pointed it out.
I remember a children's book from when I was a kid, "There's No Such Thing as a Chanukah Bush, Sandy Goldstein," which had a Jewish girl who wanted to be able to have a Christmas tree like her classmates, and tried to convince her parents to let her have a Christmas tree under the guise of a "Chanukah Bush." Anyway, she gets to go to a Christmas party at one point and asks her grandfather why that's okay but a tree isn't, and he explains the difference between helping other people celebrate their holiday, and being happy for them, and celebrating something for yourself. He considered it okay to go to the Christmas party his union was having in order to celebrate *with* them and wish them well, but having a tree of your own was celebrating it for yourself, which they wouldn't do because they were Jewish. I think the Christmas card falls under the same rule... he should have accepted it as part of their celebration and been happy they thought of him, without projecting his own "we don't do this" onto it.