Dunno if there is anything you can say. Some people just get frazzled quicker then others. I thrive on the pressure myself, and whatever flaws may arise, I roll with the punches and make mental notes for next time.
My example was Christmas dinner. Turkey was done over an hour before everything else (and before everyone came to eat it!). The acorn squash wasn't quite done. As people arrived and brought stuff, my kitchen turned into Grand Central Station. The latter was easy enough, as I allowed those in that needed to prep stuff or drop off stuff for the dinner, and threw everyone else out. Even those with a pass to stay were only given the minimal required time. As for the squash, it was done enough to pass, but I would keep it in a bit longer. For the turkey, I just pulled it out and kept it covered until about 20 minutes before time to eat, then put it back in the oven. Was perfect.
My point? What's done is done. If you do your best, what more can you ask for? No one could have said anything to me to help. The only thing to really say to your friend would be to ask if it was really worth her stress level, and could there be alternatives to reduce the level (i.e. personal chef for food, cleaning service for housecleaning, etc.). Then again, if she's meticulous like Mrs. Big Dog is and wants everything done just perfectly, which can only be done by her, there's nothing to say but best of luck, I'll help if I can!
Integrity & Honesty
have been sacrificed on the combined altars of Control, Political Correctness, and Convenience