You know, when you are absent-minded, his project progress will be damaged because you don't devote yourself wholeheartedly. This is not the result that everyone wants to see. So, if you really regard him as a friend, you should help him recruit someone who can replace you, and then tell him that you are going to resign, and you have helped him figure out a way to ensure the smooth progress of the project. This is very safe and will not affect both sides. As long as you let the other party know that your resignation is not due to the breakdown of friendship in your, nor to your subjective emotional influence, but only due to objective reasons, then as an adult, everyone must be acceptable. Most things in the world are communication problems. How to communicate, how to take care of each other's feelings and face, and how to handle things smoothly and properly are your tests, but they are a necessary lesson in the workplace.