Think of it as "giving help to". Same situation as "danken" --> "we are giving thanks to him".
These verbs always take the dative case: begegnen, danken, folgen, gefallen, glauben, geho(umlaut)ren, gratulieren, helfen, kondolieren, schaden, passen, gelingen, trauen. I copied this from a high school book, so I think it's reliable
This site explains dative, lists prepositions that can use dative and lists verbs that require dative. Just scroll on down. http://coerll.utexas.edu/gg/gr/cas_07.html
Here is another list of dative verbs. http://german.about.com/library/verbs/blverb_dativ.htm
http://german.about.com/library/verbs/blverb_dativ.htm has a good list too
From my impression, I think "Mann" only means husband when it is being possessed. For example, "ihr Mann" would mean "her husband." But don't quote me; I would check with a native speaker.
As christian said higher up, "helfen" always takes the dative case, and the dative case of "der" is "dem."
What's wrong with "We helped the man"? Is it because "helped" is past tense?