All right. Several posts have touched on truth here, but I feel a couple of things need mentioning.
First, the one thing you can be certain of is this has nothing to do with the one thing you mentioned: "My wife, however, has shown no desire to stay attractive." This is almost universal for women: they want to be attractive, especially to their husbands. There IS a deep emotional reason behind the drastic change, and she almost certainly hates it as much as you. This underlying issue is something that needs to be spoken honestly about.
If you care about her, you will realize that this is not a "I have a fat wife" problem, this is almost certainly an "I have a hurting and depressed wife, who needs to talk" problem. There are no guarantees, but there's a very good chance some good, honest communication will work wonders.
Lastly, as far as how to approach this. Never, EVER say anything that could ever be taken as, "Honey, you're fat, and it's a problem." What you say is something like, "Honey, you seem depressed. What is going through your mind?"
Treat the weight gain as what it is: a completely secondary manifestation of the actual problem. Once that problem has been resolved, the weight issue will begin to repair itself like magic.
What you have to decide is if you care about her enough to take on the effort to really resolve the issue. If you do, you have a very good chance of good things happening. It really can get better.