I want to serve sweet potato fries at a party (80 guests) Is there a way to precook or partially cook them the day before, so I can just reheat them on the day of the party to serve warm and crisp?
As user36802 noted, cooking fries normally requires blanching the fries beforehand, which will par-cook them.
You can also check out this Food Lab article; in it, Kenji Lopez suggests parboiling the fries, then drying them and frying them once, followed by cooling and freezing them overnight. His recipe continues to fry the fries a second time before serving, but you could theoretically reheat them in the oven instead. I would assume that most of the moisture has escaped the fries after the first fry, so ideally you wouldn't notice much of a texture difference between twice-fried and fried-then-baked fries, but I haven't tried that method myself.
Also good to note that his recipe is for regular-potato fries; however, the technique should work just as well for sweet potato fries (in fact, I'd assume that since sweet potato fries tend to be more moist than regular fries, any texture difference from frying then baking should be less noticeable).