Oh, I don't think she committed to "providing half the food for the entire meal."
I think she committed to "bring a 9x13 pan of butternut squash, a large salad, some candy and some cheese."
Betty can take care of the rest--she's a grownup!
And if it's a disaster, then POF can take her family home and feed them food at their own house. I'm sure they won't hold it against POF if there's not enough food. (I'd be tempted to make sure everyone ate a very late and very large breakfast, just in case.)
POF is just "helping out a little," and as long as she has been clear about what she is bringing, the TRUE hostess has ALL the responsibility of making sure there's enough food for her guests.