First off, I'd want to ask the coworker who told her about the bf saying antisemitic crap at the party if anyone took him to task. If not, then Jan's bf is not the only problem she has to deal with. She needs to be asking loudly, how come no one took this guy to task when he said that if no one did. This will let Jan know this is a problem she needs to start dealing with. Secondly, telling Jan that she will have to make other arrangements--which means not going to the party herself--exiling herself is not an option here--what terrible advice! I think talking to Jan and letting her know that her bf saying antisemitic stuff, especially at a work party is making LW uncomfortable is the thing to say. That puts the ball in Jan's court to see how she wants to deal with it. It's not all up to the LW.

I"m a Jew and had a coworker say something antisemitic in front of me at a job several years ago, and fretted a day or two, but then mentioned it to her--she didn't realize what she said was antisemitiic or know I was Jewish. She took a couple of days, but then let me know she got it and apologized. I appreciated that. But she wasn't a raving antisemitic conspiracy theorist like the LW's coworker's bf. But if LW doesn't speak up, who is going to get these people to check their bigotry?