IMO, not leaving her OSO alone for Christmas is a great reason to "choose" him. I mean, you have family and friends to spend Christmas with right? Is it really the end of the world if she's not there? My husband is going on a cruise with his daughter for Christmas this year, I won't see him until the day after boxing day. I'm really excited for him that he's doing something fun with someone he loves.
Regards to inviting them both for xmas but not disclosing the poly situation... just trying to figure out how you would present this situation to your family then? "This is my girlfriend, and her....uhm....friend...." ? Just seems it would be weird. And then you're asking him to watch you and your gf be romantic the whole weekend, I assume you'll get to sleep with her and he'll take a couch or spare bedroom, adding insult to injury...
Personally, I couldn't do it. I hate the idea of being someone's "dirty little secret" and unable to disclose the relationship status just to avoid awkward situations, especially if it's someone else's awkwardness...
Put it this way. Would you be willing to invite her as "your good friend and her boyfriend" and let them share a bed for the weekend, etc?
Because really, being "with" your girlfriend in someone else's house with someone else's family, watching everyone be close and family-like together, and even at night your girlfriend is sleeping with someone else... well, in that situation I would probably feel even more lonely than actually being at home alone.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker