I have no idea how it works in Iowa, that being said party hunting is legal in some places. Up here I can fill another hunters tag IF the other hunter was out in the same woods hunting at the time of the kill. If hes nursing a hangover back at camp its illegal, but if hes on a watch hunting when the kill is made its legal.
Buying a tag for a non-hunter and using it is definately illegal.
BHF is right that its hard to prove (a crime) after the fact, but here in a one tag area it sure is nice to be able to continue hunting after tagging out, you just have to convince a friend to go out with you (and possibly forfeit their tag).