There is a grief forum here. It sounds like what he is experiencing is very normal and unfortunately will have to pass thru this phase. The best way for him to do that is to vent, talk about it. He does not expect answers, just needs someone to listen to him and I think if you send him the link to the forum here, it would be great. He will realize that others are experiencing the same emotions. I think you are great! Helping like this.
If you go to the top of this page and click "forums" there will be a box in the upper left hand corner, type in "grief" and this will take you to a website that deals with this type of loss. Once on the website yourself, you could copy and paste the web address into an email and send it to him. This way all he has to do is click on the link in your email. He needs someone he can talk with right now, and it appears that your are it. But he will find lots of support on that specific blog. He obviously feels very comfortable confiding in you, and looking to you for support. He may eventually need professional help, but right now he just needs to grieve any way he feels is best. You don't need to be a professional to help him, especially since you knew his wife so well. You can tell him that his wife would understand his gried, but would not want him to give up. His daughter still needs him, and his wife would want him to be there for her and any other children they have. Tell him to write to his wife, this is very therapeutic, whether on the computer, or on paper, putting your feelings in writing just seems to help. Just listen, and encourage him. I think he will eventually be okay, but this is such a terrible loss, he needs time. A good friend of ours lost his wife after 37 years of marriage, and we truly didn't think he would make it through it. He sat in front of the computer typing, and sobbing. He is doing very well today.