When I was in DBT, one of the group leaders told us about a few people that have been "recovered" of BPD from DBT. I think it was to give us hope.
In my opinion, I don't think anyone is ever completely cured of it. I depends on whether or not you believe personality disorders are permanent.

I mean, I guess there are some people who start off with all the symptoms of BPD and then later on all the symptoms go away, but I don't know if it's purely because of DBT. There's a lot of other factors that go into it too.