I would imagine that the parents would either bring their own booster seat, or figure their child can sit on a regular chair. We use booster seats at home, but my newly-3-year-old can sit in a regular chair in a pinch, so I don't usually bring the booster seats when we go out or to somebody's house. I would probably have brought one when she was a very young 2-year-old, though, so I'd say that would depend on the child's abilities.
I absolutely wouldn't recommend that you go to the effort to buy plastic dishes or toys. I think that most parents would probably bring a few toys with them. However, I do like the suggestion to put a few things along the lines of a spatula, a wooden spoon, some plastic storage containers, etc. into a little bin for him/her to play with. Toddlers will often finish eating before adults, and that way he/she has something to play with, if the parents didn't bring toys, while the adults talk. You certainly don't need to do anything like that, and I think most parents would bring toys along, but it's a fairly simple idea to have ready just in case. If the parents bring toys, you don't even have to put the bin out.
I think it's not a bad idea to have a paper plate and plastic silverware ready, if you have some, just in case the parents don't bring dishes. But if you don't have those, I'd wait and see what they bring. If they bring nothing, and you don't have any plastics, consider letting the child use a napkin or paper towel (or a few) as a plate. At that age, using the hands to eat is no big deal, and you can offer a spoon just in case. If they didn't bring a cup and all you have is glass, consider letting the child use a mug (has a handle) that maybe isn't a special favorite. Or even something like a shotglass, which is thicker glass for its size, so less likely to bring if dropped.