@augusta,
Hi Augusta,
(I'm not a psychologist - don't know anyone on this forum that is) My opinion on this is that it is not something you can help him with (easily).
It's something he needs to talk work out for himself - with help, or by himself (with a professional is usually easier).
What sort of guilt is it - does he feel disloyal? Does he feel guilt over the circumstances in which she died? Does he feel that he has something to make up for that he never had a chance to? Or some other.
If he wants to try to work it out by himself - I'd probably ask him to put all his thoughts down in writing (handwriting, not typewritten). The subjects could include : His thoughts on his deceased wife (what he treasured about her, her strengths, her weaknesses, the things that made him love her), what their relationship was like (how he treated her), how it ended, any regrets, what he believes she would think of him moving on, and whether she would want him to move one. Etc.
It might at least help him sort out some of his own thoughts and feelings.
You could also ask for help on some psychology sites. I'm sure there must be some out there. Good luck.