I believe good ground manners are essential for having a well-behaved ridden horse.
These are some of the things my Mum and I insist our horses do when on the ground.
1. Lead correctly. This means not barging ahead when being led out of the stables, not invading our space whilst leading, and generally being led co-operatively. They should walk beside us, but not right on top of us. Just work on it whenever you have to lead him somewhere.
2. Standing still. This takes a bit of time, but tie him up, and just leave him there. But still be around, so you can get to him if he get's fidgety or spooks. If he starts to fidget, say woo (or whatever calming word you use) and tell him when he's a good boy. You need to pet him when he stands still, so he learns its a good thing.
3. I don't know if your friend does led classes, but teach him how to set his feet square. This can take a bit of time, if he's new to it and a bit cheeky, but its a really good thing for shows. Pick a hoof (I pick the right hind), this is your base hoof. You want the left hind to be square with it. And make the front hooves square and in porportion with his hind legs. I could go on and on about this, but that's just the bare basics. Look it up on Google, if you want to know more detail. Remember to reward him whenever he does good.
4. Showmanship skills! Even if you don't do western, its a really good thing for a horse to be able to do. So teach him how to pivot (look it up if you don't know), how to stop really quickly from a trot, how to transition properly into a trot whilst being led and such.
5. Grooming manners. Be persistant when picking up his feet. If he tries to fight you picking up his hoof, don't let go until he stops. You are the leader, and you need to be in charge. He needs to respect your space, and not intentionaly (sp?) do anything to harm you whilst being groomed.
The little things are the things you need to work with. Even if your just leading him to the wash bay, do some littles exercises with him. Consistency is the key!