Delirium: Given the patient's sudden onset of altered mental status, disorientation to time, and confusion about recent events, delirium is a strong consideration. The fact that she has a clear speech, steady gait, and no focal neurological deficits also supports this diagnosis, as delirium often presents with fluctuating cognitive impairment without significant motor or speech abnormalities.

Dementia with Acute Worsening: Although the patient's friend reports that she was last known to be well the night before, it's possible that the patient has an underlying dementia that has acutely worsened, leading to her current presentation.
Infection (e.g., Urinary Tract Infection, Pneumonia): Infections are common in the elderly and can cause altered mental status, especially if the patient has a urinary tract infection or pneumonia, which might not always present with typical symptoms such as fever or cough.
Medication Side Effects: The patient could be experiencing side effects from medications she is taking, which are causing her altered mental status. Elderly patients are more susceptible to the cognitive effects of many drugs.

Stroke (Ischemic or Hemorrhagic): Despite the lack of focal neurological deficits, it's crucial not to miss a stroke, as some strokes, especially those involving the posterior circulation or small vessel disease, might not present with typical signs like facial droop or speech difficulties. The family history of stroke increases the concern.
Hypoglycemia or Hyperglycemia: Both low and high blood sugar levels can cause altered mental status and are easily treatable. Given the patient's age, she might have diabetes or another condition predisposing her to glucose imbalances.
Subdural Hematoma: Especially in the elderly, even minor trauma can lead to a subdural hematoma, which might not always present with immediate symptoms but can cause delayed onset of confusion and altered mental status.

Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause rapid cognitive decline and altered mental status, though it typically presents with myoclonus and other specific neurological signs.
Hashimoto's Encephalopathy: A rare condition associated with Hashimoto's thyroiditis, characterized by altered mental status, seizures, and stroke-like symptoms, which is treatable with corticosteroids.
Wernicke's Encephalopathy: Caused by thiamine deficiency, often seen in alcoholics, but can occur in other conditions leading to malnutrition. It presents with confusion, ataxia, and ophthalmoplegia.

Complete Blood Count (CBC)
Basic Metabolic Panel (BMP) including glucose
Liver Function Tests (LFTs)
Thyroid Function Tests (TFTs)
Urinalysis
Blood cultures if infection is suspected
Imaging: Non-contrast CT of the head to rule out stroke or hemorrhage
Consider MRI of the brain if CT is negative and clinical suspicion for stroke or other structural lesion remains high
Electrolyte panel
ECG
Toxicology screen if substance use is suspected
Lumbar puncture if subarachnoid hemorrhage or central nervous system infection is suspected
Thiamine administration if Wernicke's encephalopathy is considered, given its potential for rapid improvement with treatment.