"They're Reform Jews and I doubt they'd take offense at getting a Christmas card"
There again, why would anyone ever take offense at receiving a loving gesture in whatever religious form it takes?
My guess is that most Jews who receive a Christmas card react to it in the same manner that most Christians would react to receiving an 'Eid ul-Fitr' card from a Muslim coworker.
IE - Thanks for the sentiment (wrong holiday, though).
:-)