I usually use "deck" to mean a short paragraph below a subhead that briefly introduces an article. As far as subheads go, yes, they often look best with the first line longer, but there may be a specific reason to break it differently: for meaning, to allow a narrower column, to allow a larger font, to avoid splitting a name, etc. So it depends on the specific instance as well s the overall style of the publication — the choice is a stylistic one as well as an aesthetic one.

Your colleague would probably like to see both lines as long as possible, as white space was considered somewhat precious, if not downright effeminate, in a certain style of news design.