of course the goal is not to let anyone leave the start-up, the assumption are: 1) you team member doesn't put his energy on the project, 2) you team member can't execute as well as he is committed to. 3) you have tried every single possible way to improve his performance but 4) he just refuses to change.

in this extreme case, you have to take eaction to avoid a deterioration within the team even if he is your friend in years.

of course, you guys might still be friends in life, life is not just about start-ups anyway.