I'm leaning toward the Mother dying of some disease. I'd have to rewatch the scene, but I'm pretty sure the dialogue was "What kind of mother doesn't make it to her own daughter's wedding?" The gender specific nature of the comment makes me think it's about the Mother rather than Ted's mother. They know that she won't be there when their daughter gets married. I really hope I'm wrong, though. Yes, I know it's just a TV show, but after all of this, it just doesn't seem remotely right to leave Ted alone and heart-broken again.