How do you teach a kid to wipe their butt properly?

I know the title is funny but I’m a preschool teacher and I had an older student shart himself today poor guy and I told him to wipe himself and clean up while I got him new clothes and when I came back he had like a toilet paper tail and I asked what was wrong and he said “can you help me because my mommy says I don’t know how to wipe my butt.” So I cleaned him up with baby wipes. But he’s 4 and I’m assuming by how he just...left a wad of toilet paper between his cheeks and waited for me he doesn’t know how to wipe himself. I am involved in potty training younger ones but I was surprised that this is an issue for an older kid. How can I instruct/help him if I happen to know he’s pooping again or asks for help? By the way I am not a parent.