Hey, I think I understand your friend. He believes that by planting on June 1, the plants will grow until the summer solstice (June 22) and then start flowering, hence they finish by the end of August. Theoratically it works, the problem is that the solstice effect is a much slower change of the photoperiod than the indoor switch from 18 to 12 hours of light per day. The plants need some time - a month - to recognize this, that's why they finish at the end of September, if you're lucky. Try to explain this, hopefully it will convince him. Have a nice crop!