I've done the foil thing, it still gets too hot. cut a slot into one side of a box and move the pot in so the box catches the sun. No one said growing weed needs to be pretty.
The alternative is get a 1500 gal smartpot, that will protect your roots by its volume.
To reduce heat he could cut large holes at the bottom sides of the box to increase airflow, similar to having grow tent air flaps at the bottom open to increase airflow. Then again heat is also based upon climate unless your plants live in a vault, infact I had to add a portable AC, cause they were going over 30°C, even if I have a decent airflow, I don't want to ruin my buds
When I notice these changes I got to act fast or they will suffer and that means i'll suffer with less yield , it's a vicious circle
Heat waves are BAD, Almost lost a full crop cause of it, I was lucky. Another heat wave incomming this week, so for me I have to double check things.
Best to be safe rather than sorry