You are obviously extremely terrified and in shock. There are several actions that you can take to make things right with Harry. You can show Alan that he can trust you again, tell an adult and tell them why you did not tell anyone, and just be there for Harry in his time of need.
Harry is probably having a lot of trust issues because he thought that you would be a good friend and stop it but you did not. To make it right you should come up with ways to show Alan that he can trust you. You made a mistake and you need to learn from it. Sometimes, there are consequences when making a mistake but that does not mean you should not tell anyone.
Now, you need to tell an adult. Nothing can help Harry unless you stand up for him and tell someone.
Harry is probably having a lot of trust issues because he thought that you would be a good friend and stop it but you did not. To make it right you should come up with ways to show Alan that he can trust you. You made a mistake and you need to learn from it. Sometimes, there are consequences when making a mistake but that does not mean you should not tell anyone.
Now, you need to tell an adult. Nothing can help Harry unless you stand up for him and tell someone.