All the actual beaches are basically the same. Isle of Palms and Sullivans are more upscale than Folly. All 3 are barrier islands, roughly 6-7 miles long by .5 to 1 mile wide. Each island has a small business section with restaurants, shops, etc. The rest of the island is residential. There are no high rise buildings, amusement parks or that sort of thing.
At any of the beaches you will need to be there by about 9am to avoid sitting in traffic an hour or more. You need to leave by about 2pm to avoid the same issue as people begin leaving.
Young people like your niece prefer Folly Beach over Sullivans or Isle of Palms for a day trip. I like it best when the tide's out - the beach is quite narrow during the two hours before and after high tide. But high tide is right at noon that day, and there's nothing you can do about it. It's going to be too cool to swim, so walk on the beach and out on the pier (free unless you're fishing). Folly has a 3-block commercial strip with Tshirt shops, beach souvenirs, restaurants, etc.
Friday traffic to the beach won't be bad until 3PM - that's when the afternoon commuter traffic starts to clog from downtown across the James Island Connector - and shouldn't be bad for your return from the island.
Folly Beach has its own "funky" vibe, with lots of "cool" restaurants and shops that appeal to young people. Beach at southwestern end, where the county park is located, is a good swimming beach. In the center of the island, there is a long fishing pier. Toward the northeast end, is "The Washout", the areas best surfing beach. Surfing lessons are available if your niece is interested.
Yes, I believe that the thing about Folly, for teen girls, would be the little shopping area, funky and cool.
There are restaurants that will make Mom/Auntie happy, as well. Find a place to perch yourselves and let her go!
Thanks for all the great suggestions and advice! I've had a few chuckles as my niece is 30. :-) I'm sure no matter which beach we visit, it will be fun!
Well, then, the 3 of you can find a perch to share!!!
I've not been there, but Pier 101 is oceanfront at the public pier, should be nice for a beverage after beach and shopping.
Or, you could hit Bowen's Island for dinner, on the way back from the beach, beautiful marsh views. They open at 5:30, best time to be there to not have to wait in line.