I would call her up and say that I was hoping that she would be able to to get the supplies for the margaritas, and without stopping to breathe, I would start reading the list of supplies. I would tell her that I had meant to call earlier, but with all of the things that had to be done, I just
didn't get a chance to call sooner.
It's better to get some help than no help. I would also let her pick up any flowers or last minute noshes.
If she is not a person who entertains, maybe she is oblivious of what
is involved in throwing a party. Give her a chance to help.