Emily rolls her eyes
Trapping an immortal being isn't "taking care of it". That's just taking a problem and saving it for later. You think the Lich doesn't have folks on this side who would help bring it back? Or that it wouldn't be able to come back on it's own given enough time? Eternity is eternity. The lich can and will escape someday. Maybe not today, maybe not tomorrow, maybe not for the next thousand years, maybe not even the next million, but trapping an immortal being will not be permanent no matter how you do so. Immortal beings literally have all of eternity to escape. Hell your witch friends even said he isn't escaping anytime SOON. Odds are, they both know that he will escape someday, just not soon. Besides, why trap them in Purgatory, one of the circles of Hell would be a better choice.
Regardless, it's phylactery is gone and so is it's physical body. When he does come back from purgatory, he will be killable. I just wish those two witches had waited a bit before banishing him, we probably would have killed him by now rather then just have him trapped. Just help my take the coffin back to the docks. I'll stow it in my ship and leave. Gotta put it back where it belongs.