Besides bringing the right equipment, the most important thing for you to do is to observe the fight and learn as much as possible about his opponent. Did you notice that the guy drops his left when throwing a rear straight? Tell your friend to counter with a right straight. Whatever information your friend can use that he didn't learn by himself (it's hard to notice stuff like that when everything is going so fast). You tell him this between rounds ofcourse.
You can also help him to attack at the right moment. You have to practise this alot but if your friend manages to learn to hear only your voice when fighting, you can tell him when to attack. Your voice has to work like a button. When you say attack, he attacks immediately. You might see something that you know will benefit your friend if he attacks at that exact moment, but he himself might still be dancing around in the ring/octagon for whatever reason. So when you yell attack, he attacks without hesitation. And the opponent might get caught in a flurry that send him to the canvas.
Though you have to talk to him and let him tell you what kind of cornerman he wants you to be. Do he want to hear you cheering for him continuesly? (yeah way to go, nicely done, you're the best) Do he want you to only open your mouth if you have something useful to say? (relax, breathe, work those low kicks). Every fighter is different. So talk to your friend and ask him what he wants you to do when he is fighting. Hope this helps.
Take care.