I heard from a gardener, and she says through direct observation, that it is bad for plants to be watered in the morning on hot summer days, not from water droplets focusing heat (a myth many internet articles like to point out), but because the new growth tips that are fostered by morning watering are not able to survive the heat and then would scorch. She says that you should always water in the late afternoon/evening so that the new growth being fostered has the maximum amount of time to toughen before the heat hits. Most articles say the opposite, that morning is the "very best time." I can see concern over mold on cucurbits, maybe morning is better for them, I don't know, but she sounds right, but it's difficult for me to find research that talks about this concept.
Certified GKH Gardening Expert
This is an interesting question and the topic is under debate among gardeners and farmers. I am not sure about your friend's idea- it may be plausible with some plants, but with sufficient water and with a plant adapted to the climate you have, I suspect the new growth should be fine.
Watering at night is thought to contribute to fungal growth on a wide variety of plants, not just cucurbits, but watering in the mid-late afternoon may allow enough evaporation to prevent this. To avoid this potential problem, you can try to direct the water as low as possible to the ground so that most of the leaves do not get wet.