Was the someone staff or a random stranger? Either way I'd take all advice picked up in pet shops with a grain (or bucket) of salt, its kinda scary how often its incorrect.
Not all hamster species are prone to diabetes and even animals of species that are prone to diabetes probably won't have any harm done if they're fed the occasional unhealthy treat. I'd avoid giving a diabetes prone species giant chunks of apple or sugary treats on a regular basis but in moderation I seriously doubt sunflower seeds will do much harm