Oh! I almost forgot! He has already tried bullet. It is helping him a lot, but we need something else.
How long from GM to patzer?

Spend half an hour playing antichess and other variants. Then study tactics straight after - it's impossible to switch back to thinking normally.

Walking around like a zombie with a smartphone while texting and doing Facebook is a good way of reducing one's intelligence. I suggest your GM friend tries this.
Also wearing jeans that show off their 'stylish" underwear at the back seems to work for some people as well.

Join the witness protection program and move to Idaho. He could also watch some Backyard Professor videos.

Best way to destroy your chess is play Bughouse right before every round of a tournament! That will mess up your head big time, and you'll start failing.
Problem is though, there are rating floors, and once a GM, always a GM.

Spend half an hour playing antichess and other variants. Then study tactics straight after - it's impossible to switch back to thinking normally.
THis is a great suggestion.

Suicide is probably the most effective method. Dead people are usually not able to play chess very well. Not to mention, most of us patzers recognize that bothering the dead for things like game reviews, books, etc is a fruitless endeavor.

A friend of mine has decided that he no longer wants to be a GM due to several factors: psychological pressure to perform well in every tournament, patzers asking him to analyze random games (of course only the games they win), patzers asking how to become a GM in a femtosecond, patzers asking him to write a book, etc.
So he has tried disappearing for a while, but it has not worked, the patzers have found him anyways. That is why he is looking for a way to become worse at chess.
Feel free to suggest your methods to become a patzer again.
If he finds a good job, he will forget all GM chess skills.
A friend of mine has decided that he no longer wants to be a GM due to several factors: psychological pressure to perform well in every tournament, patzers asking him to analyze random games (of course only the games they win), patzers asking how to become a GM in a femtosecond, patzers asking him to write a book, etc.
So he has tried disappearing for a while, but it has not worked, the patzers have found him anyways. That is why he is looking for a way to become worse at chess.
Feel free to suggest your methods to become a patzer again.