A person with low blood sugar is confused, sweating, unsteady, slurring their words, sometimes aggressive, and smells of nothing in particular. They have been arrested for it. They have died in cells for it.
Assume hypoglycaemia is possible in anybody behaving like this, and especially in anybody who has diabetes, is wearing a medical alert bracelet, or has an insulin pen or pump on them. Look for the bracelet, the pen, the pump, the card.
If they are conscious and can swallow safely: something sugary, now — glucose tablets or gel, fruit juice, a sugary (not diet) soft drink. Then, once they improve, something starchy: bread, biscuits.
If they cannot swallow safely, are very drowsy, or are unresponsive: nothing by mouth at all. Recovery position, call the emergency number, stay with them. Never put anything in the mouth of somebody who cannot protect their own airway.
High blood sugar builds over hours or days — thirst, passing a lot of urine, deep sighing breathing, a smell of acetone, drowsiness — and also needs an ambulance. If you cannot tell which it is, treat for low: sugar will not meaningfully worsen a high, and withholding it from a low can kill.
Fainting — a simple faint — recovers in under a minute lying flat with the legs raised. Anybody who does not come round quickly, who faints during exercise or while sitting or lying, or who faints with chest pain or palpitations, needs an ambulance rather than a chair.
Source. World Health Organization, Diabetes fact sheet, https://www.who.int/news-room/fact-sheets/detail/diabetes ; European Resuscitation Council, Guidelines 2021, First Aid section, https://cprguidelines.eu/guidelines-2021
What this is not. Reading, not certification, and not a diagnosis.