As long as the primary care doc is sure there's no risk of an intestine popping out through the hernia and becoming "pinched" in a way that blocks the intestine or cuts off blood supply, I guess that's OK.

But as a guy who tries to think ahead and use all available diagnostics to prevent a problem, I would inquire whether imaging or some other method can confirm it's OK to wait.

FWIW, I had an umbilical hernia repair about 7 years ago (before cancer), and it was a rather painful first week of recovery. Now that your husband is 6 months post-Whipple, I assume the pain from that surgery is no longer an issue preventing a hernia repair. But if he's still struggling with chemo, I wouldn't compound the suffering with an unrelated surgery if it can safely be put off until later.