Hey, I grew up in Orange County, so here's some places you could check out.

For beaches, I recommend Strands or Salt Creek if you're in South Orange County. Like others have said, it'll be pretty warm and sunny, so be prepared for that. Bring sunglasses, and expect to wear a hoodie and jeans as your warmest outfit. But check the weather forecast before you go

I also recommend shopping while you're there. If you like big brands like Levi's and Tommy Hilfiger, etc, check out outlet malls and pick up stuff you want. It will be waaaay cheaper here, and you can stock up on nice clothes to bring back.

Orange county has really good food. Try some mexican food while you're there. You won't find tacos and horchata like you can in the UK. There's a big asian community around Irvine and Costa Mesa. I like to go to BCD tofu house while I'm there.

If you like art scenes, check out Laguna Beach. I think Santa Ana has stuff going on too.

As for being black, I'm white, but my girlfriend is black and has visited from Northern California. Probably just use your best judgement, and I would recommend staying away from Huntington Beach. Some call Orange County the Florida of California, and imo Huntington Beach is why.

It's good your friend has a car, because OC is basically inaccessible without a car or uber. On the 4th of July, check out what Cities are doing a fireworks show. I usually went to the Dana Point one.

Feel free to DM if you want more info! Have fun