If it isn't sacred to you in the sense of being used in your personal practice, and the owner has passed and has no need to detach the altar from their practice in their mind, I wouldn't stress too much about needing to duplicate some exact ritual. It's not like there's any universal standard used by people not part of an established tradition, so we can't know what his practice was exactly.

Instead, I advise just being generally respectful and doing what feels right to you as a show of respect and a farewell to your friend. You are the one doing the ritual now, if that makes sense. You know what you wish to say, and you are the only one who knows the most meaningful way for you to say it. I am sorry for your loss, and wish for peace for you both.

Here is a process Speak gently to the spirit of your friend and explain that you are helping his release from this life. Ask him to show you in way that you can understand his wishes for his property. He may appear in dreams or some of his old friends etc may show up and ask about things

For altars

Consider which beings of endings, cleansing and purifications you work with

Ask them to show you in a way you can understand how to clear that altar so it is no more than its original materials

Once it is cleared you may want to burn it and scatter the ashes so that it returns to the universe

You do not owe Ahriman anything except respectfulness...do not make offerings as that may create an unwelcome link...

I would just quickly explain that I'm going to dismantle the altar, thank whatever spirit is listening through it on behalf of the deceased user, free the spirit of its duties and perform a cleansing ritual. What is left requires no further occult consideration.

This was the plan. Dismantle with respect. Thank you everyone.