Idk. I got my horse when he was only almost a year old and to be quite honest, I think it depends on the horse's actual demeanor whether or not her friend will be able to 'handle' the horse itself. I mean, if it's got a gentle and willing nature to it, yeah, he'll be fine, young or not. But if it's got attitude and loves to put up a fight, then perhaps he's out of his league. Don't let age fool ya, look for personality and how willing they are to work with you. I know some horses that are in their twenties and still a pain in the butt to deal with. Age is a number, personality is what makes up a horse.
Another thing is that the more you work with the horse and bond with it, and respect it, the more willing the animal will be to do as you ask. Then of course, I'm sure most horse owners know such things. I just hope your friend bears patience and a lot of love because for a first time horse owner, it's not always easy.
Yet at the same time, the horse itself has been trained and worked, so I think your friend should do just fine, if he can work with whatever kinks there are with the horse and adapt. Again.. Personality is what makes the horse buyable or not.