As PP mentioned, it's not actually prime rib, but it's an acceptable cut of beef. By plain, do you mean not rubbed with any spices? I don't actually recall how it was prepped, but don't think it was heavily seasoned. I will say that they are a bit restrained with their portion sizes, which makes sense given that most people load their plates with many things, so if your father will only eat the beef, he may need to encourage them to give him more than two small slices.
I don't recall french fries on the kids buffet, but our kids don't like french fries (we'll let them eat them, they just don't like them) so I probably wouldn't have noticed them. DD5 did enjoy the mac & cheese.