"Prime rib" is a term of art, but it can vary by user. It usually means a beef roast taken from the middle 6 ribs of the rib primal. The roast may be between 3 and 6 ribs. A "standing rib" roast may be between 2 and 8 ribs.
When figuring service amounts, the first thing you have to know is whether you're cooking "bone in" or "bone out," If you're cooking "bone in" you should allow two to three persons per bone, depending on whether you want to be generous or tight. "Bone out" is also called a "rib eye" roast -- and includes the "flap" as well. For most affairs, 2.5 people per bone, or a roughly 10 oz (allowance before waste) serving, is adequate. Before waste, an 8 oz slice of prime rib is a "lady's" portion.
Chicken servings are usually figured per piece, rather than by weight. Birds are usually broken into 8 pieces. Assuming 3.5 pound dressed out birds, broken into 8 pieces, 2-1/2 to 3 pieces per guest, may be considered an average allowance.
Figuring a 2 Entree Allowance without other input:
Unless your client specifies otherwise, it's a good idea to guess the entrees will split equally, but to allow overage with both in case either is a standout with the particular crowd. Also, it's a good idea to allow more food for a buffet than a plated and served dinner. Buffet dishes must be kept sufficiently filled to present an appetizing appearance; carving stations must have a nice piece visible -- and ultimately this means some waste.
With an entree as expensive as prime rib, it's a good idea for the hostess/client to request that her guests order in advance as part of the RSVP. Otherwise, you'll either run out at the part, or you better have a dynamite prime rib hash recipe to pass along. In any case, it's a good idea to discuss the relative expense of prime rib with your client.
(Grain of salt!) Unless it's a done deal, you might want to suggest roasting large pieces of top sirloin as an equally good, but less expensive alternative. One of its advantages is that sirloin carves into thin slices beautifully, which will encourage a "some of this, some of that" approach by the guests. Before agreeing with me though, a "Santa Maria" style top sirloin grill was one of my signatures when I still catered -- so take the recommendation with a grain of salt
The exception to the 50/50 rule is something expensive vs. chicken. While a lot of people are watching their weight, "don't eat red meat," etc., some people just can't resist the luxury. And beef is, well, just so darn beefy. I'd break an average crowd down as more than 85 full servings of prime rib, and less than 80 full servings of chicken.
That's 5 full (6 bone) prime ribs, 4 full (8 bone) standing ribs, or about 54 pounds of uncooked, untrimmed rib roast, or about 50 pounds of top sirloin; and 30 chickens.
Hope this helps,