The rental bikes are easy to ride compared to other bikes, but Seoul bike paths can be crowded so maybe he has the right idea to avoid it if he is nervous on a bike.
There are a few overnight campgrounds by the river with proper camping facilities, but it's not a really good place to sleep because of the noisy and crowds (need a lot of alcohol). This isn't one of those places anyway.
You need to use day tents which are smaller and have one see-through side (people were doing stuff) and you can't have a tent up between 11 pm and sunrise. Before the rules it was a mess. It still gets trashed every weekend.