Diagnostics are a very good way to sort out possibilities. I agree with your friend, if the Dx matches the behavior then there is reason to pursue ADHD options. Males often are diagnosed with ADHD in their teens, or they have the traits and are never treated...the Dx is missed. As adults, the brain matures enough to minimize the impact of any potential hyperactivity, but the inattention and failure to complete tasks remain. (ADHD exists in several subtypes, and although ADD is not used as a technical term in the US anymore, the subtypes allow for any combination of traits, from hyperactivity to inattention to combined types.)Much research has been conducted on adult ADHD in the last few years and very effective treatments are available. These treatments, if best practice, involve medications and supportive, learning oriented therapies which reduce symptoms and teach ways to work around the inattentive symptoms. Many adults seek help through a referral from their family doctor. Almost any psychiatrist should be able to assist him, although I will admit a preference, if your husband is willing, to see a psychiatrist who also sees teens and children. They tend to be well versed in the latest medications and techniques.
I know you are at the end of your emotional rope, and I would encourage you to seek support as well. This cannot be easy for you in so many ways. But, know that treatments have been getting much better for this issue and even medications combined with nutritional changes have been used. Take heart! This is, even if it is ADHD, treatable and resolvable. Steven