So what happens to what used to be his account?

I kind of hope that banned accounts just remain in a stasis after their ban, if only for nostalgia reasons. So I can look back over it and remember some of the good times we had playing together, I realize he will have a smurf/new Main, but it would still be nice.

The account stays banned for 100-200 years, after that is it supposedly open to become playable again. So, it doesn't get deleted and you can remember the times.

It is there.
You just won't be able to play it again anymore, unless you live more than 270 years.