Wow, that sucks that he did that to you.
First thing is first... make sure your 12V system is correctly wired to 12V or a 12V converter.
The light "shells" are still good (light bar in front). So just replace those bulbs and you're good to go.
The rear LED lights may or may not have survived. Take each assembly out of the cart (they are simple to remove). And connect it directly to a 12V battery to see if it survived. If it lights, you're good. If not... just replace the whole tail light(s) assembly. They are inexpensive enough, and you probably won't find the replacement LEDs as easily as you can just replace the whole light.
The horn is a similar story. Pull it out (or leave it in, depends on how much access you have to the connector). Hook up 12V to it directly and see if it still works. If it does, good. If not... well, you get the picture. If it works outside of the cart, but won't work when connected to the cart... then you also fried the horn relay. It is located behind the dash panel that holds the key switch.
Now there are some other problems to consider. If he connected your 12V system across the 48V pack, there are also flashers and a brake light "time-out" timer that could have been damaged. I would replace your front bulbs, and be sure your tail/brake lights work (after repair or replace)... return them to the cart.
I assume you have a Club Car Precedent... When installed back in the cart, If the tail lights work, but the brake lights do not, you probably fried the timer. This is located under the floor board, roughly behind the pedal box. You'll probably need to replace that.
If your front lights work, but the turn signals no longer flash... your flasher is probably fried too. That is located behind the dash panel that also holds the key switch.
It's no surprise you didn't blow a fuse. Fuses protect against over-current... not over voltage. I'm sorry your friend made it more difficult for you. The good news is that the most expensive item: the light bar, is the one item that won't be seriously damaged. You'll just need new bulbs. I'm glad you didn't have a stereo in there!
Your friend owes you several beers. I recommend Cigar City Pumpkin, or even better...
Cigar City Jai Alai.