I would never put a new bird straight in. Best is to have the new one next to or in with the existing flock but protected so that the new bird cannot be beaten up, for example in a large dog crate within the run, add food and water and a roof against sun and rain, and let them get used to each other for a week before letting them mingle. Or a separate coop with run right next to the girls’ one. I have large runs with space for several coops (each with their own small run) so I give them a fortnight of being able to see and hear each other but not touch, your friend may have a large garden where he can place the cockerel next to the hens, and later on let him have a wander around the garden by himself to get to know the place before they mingle. When they finally get to mingle, scatter some wheat or other food around as a distraction.
As for the age of the cockerel, it depends more on his size, some 16 week olds are quite lightweight, others are nearly the size of a large fowl hen. The older, the better, but as soon as his dad starts fighting with the son. It’s nice to be able to rehome a friendly cockerel!