Soup making - should I add extra water and boil off for flavour?
My experience and the recipes I read are to simmer.  Hard boil and simmer are the same temperature so cooking time should be the same.  A hard boil can break up contents.  Hard boil also dirties your stove top.  That said even with a simmer you can sometimes need to add liquid.

If your husband wants to hard boil there is not much harm.  He is cooking.