Stray and feral cats are very resilient. They find shade or cool spots to hide during the hot days and will not be very active. Night time is when they can venture out. If you find that your yard does not have a cooler shady spot see if you can make one. Keep out dry food during the day and always be sure there is plenty of cool fresh water. You may have to feed early in the morning and later in the evening when it cools if you are offering wet food. Just be sure to not leave out any food at night. It will attract too many unwanted critters.
Here is an article from Alley Cat Allies on summer cat care