The hot weather may cause a decrease in blood pressure, with consequent feelings of unease and possible fainting fits.
What should you do if someone faints? Doctor Maddalena Lettino, Supervisor of Heart Failure Medicine at Humanitas, spoke about this in an interview.

First of all, you have to keep the person who fainted lying down, raising their legs, whatever their problem is.

Vagal Inhibition: The most Common Fainting fit

“A fainting fit induced by low blood pressure, called vagal inhibition by cardiologists, is one of the most common cases. It generally happens in supermarkets, while waiting in line, or in hot places such as churches.
In this case, the person generally regains consciousness immediately, because the quantity of blood that comes from the legs and goes back again in the general circulation is enough to restore a good cerebral circulation.
You should make the person lie down and raise their legs until the rescue team comes, and, if the person does not feel better, try to check whether there is a pulse. In fact, if it is absent you should start at once the first resuscitation manoeuvres”, Doctor Lettino explains.