There is no hard and fast rule that says 6 feet is too far. If the framing members are strong enough (properly sized beams) and extend into the house far enough (3' in for every 1' out), and it isn't rotted then it could very well be ok. Then again, I can't see it from here and the person could be right and its ready to fall on your head. If it deflects (sags down) noticably when someone steps on it or jumps up and down up there then it probably needs fixing and/or posting up. If I understand correctly there is NO outside support for the top deck, which means the second floor framing or beams carry through the outside wall and "cantilevers" out to support the deck. Usually, the main goal and benefit of this type of construction is having no forest of posts below holding the whole mess up. If you need some spot support then when you put your spiral stair in, just install a post right next to it. Any posts should have a proper footing dug and poured and incorporate a mechanical (metal) connection between the footer and the bottom of the post. Don't try to rely upon the stairway itself to carry any load. These are just a few thoughts, please have a qualified person inspect the deck for worthiness and don't take any chances. We cannot do this over the Internet.