You can outgrow people without turning them into villains. Some chapters just end.

When a friendship winds down, the mind wants a reason large enough to justify the ending, so it goes hunting for something they did. Often there is nothing there. You changed, or they did, the overlap got thin, and that is allowed to be the whole explanation. Turning somebody into a villain makes this month easier and rewrites years that were genuinely good.

Let it be quiet rather than dramatic. You can stop making plans without making an announcement or a case for it. Keep the good version of the memory, because you will want it later and it is true.