My guess would be that she's younger or older than he said. That guess is not based on anything besides the fact that she's not laying. You said he had lots of chickens, right? Maybe he lost track or got her mixed up with another? I have no idea how to age chickens. Is it even possible?
You can tell the approximate age of a hen, or more specifically you can tell if she is prior to POL or after POL. So, if they are approaching POL you just need to wait, if they are past POL then you know you have a hen taking a break for one reason or another (could be moving stress, could be she is spent, etc) and you need to decide what you want to do.
There are a lot of unscrupulous sellers out there. They will vaseline legs, combs, wattles, smooth feathers, trim beaks and toenails and spurs, to convince you that you are buying a younger bird. Also, in referencing "younger" doesn't mean the same thing to everyone. For me, "younger" means pullet (under a year) or first season. Given that hens often live 5 years or more, "younger" can take on a lot of meanings.
Here is what I have found in my searches on the subject (all unsubstantiated, but from experienced owners):
Prior to point of lay:
If the combs and wattles are still undeveloped or are developed, but on the barely pink side, they are young.
Legs should be shiny and without scales. Toenails will be short (of course, that requires that you have seen what long toenails on a hen look like and assumes the seller didn't cut them).
Vent will be "round".
Will investigate the nest boxes, will often "squat" when you approach.
Post point of lay:
"Find the vent. Then using your index and middle fingers find the two pointy bones below and to each side of the vent. If you can put your two fingers between those pointy bones the hens are laying, but you still don't know the actual age."
''If the pullet is just starting to lay, you will be able to place one finger in between these pelvic bones. This space will widen even further as the laying process increases, until eventually you will be able to place two or more fingers between the pelvic bones.''
"Another common and fairly accurate way is to inspect the vent, this method will not tell you their age in months but will help with their egg laying ability. Just inspect the vent of the hen if it is pink and moist the hen is still a good young layer, if the vent is whitish and dry the hen is older and probably not laying aften or well."
Vent will be elongated/flattened (smiley face, I know, I know, odd, but that is how it is described).
When they first start laying for you (after the move-in stress ends), is the egg small (like a pullet egg) or large (like a hen egg).
"Take note of the way the chicken molts. A chicken under a year old will not molt out its chicken feathers." (you won't see this until fall though).
Several seasons along or spent:
As they age and approach being spent, they "bleach out" in the legs/combs/wattles.
Long toenails, especially on the accessory metatarsus indicate advancing age (several seasons).
Inspect the shanks. They get thicker and rougher looking as they age. (I would look very carefully for leg mites if the scales are lifting, but I can say my older (4 years) hens do have pretty rough looking legs without any mites).
"Before beginning to lay eggs, a hen will retain her yellow color. This color gradually diminishes, beginning after a couple of weeks, around the vent, eyes and earlobes. After a couple months, the beak will also lose it's color, followed by the legs and feet, after about 6 months of egg production. The yellow color will return at the end of the hen's productive life or after a moult, in the same order in which it disappeared."
