Is reheating partially fried fish safe?

It's absolutely safe, and many professional cooks do this sort of thing all the time. Check out the ordinances for your local health department— some require a variance for using time rather than temperature as a food safety measure– but most are fine with par cooked meats being in the danger zone, usually defined as 40 to 140, for up to 2 hours if they get thrown out right away. If you par fry and cool the fish, then you should be bacteriologically safe for much longer, but it would probably be of pretty poor quality when you fried it. Nobody wants to eat an oil sponge. Keeping your oil temp up will help you keep it less greasy.

The Danger Zone... This refers to the range of temperatures at which bacteria can grow - usually between 40° and 140° F (4° and 60° C). For food safety, keep food below or above the "danger zone." Remember the 2-Hour Rule: Discard any perishables (foods that can spoil or become contaminated by bacteria if unrefrigerated) left out at room temperature for longer than two hours. When temperatures are above 90° F (32° C), discard food after one hour.

Though it varies from jurisdiction to jurisdiction, that 2 hour period usually starts over when the food is brought back up to finishing temperature... so if you par cooked it, held it for almost two hours and nobody got it, you could bring it up to temperature and then have another two complete hours to do something with it, or cool it and store it. Not sure what good prefried fish would be though.

You could avoid this whole thing if you had the extra time on hand for someone to butterfly the cuts so you could just fry them quickly to order.