How do I handle someone who wants to talk about problems on special occasions (such as during a co-worker's baby shower or company party) when I just want to enjoy the moment?

I know the best thing is not to be alone with that person, but what about the times I end up alone with her in a restroom, or when some people leave the table, or I walk to my car afterwards and she happens to have parked right next to me?

Should I say,

"Let's set up a separate time to talk about this" (but I wouldn't mean it because I don't have the emotional capacity right now to listen to her problems)

or

"I just want to enjoy the moment" (but that sounds really harsh towards someone who is overwhelmed and desperate to vent)

I don't want to be selfish when someone is dealing with problems, but I also really want to enjoy special occasions for what it is.