If he/she's had any intellectual input at all he/she should be on the author's list.
I'm not sure which field you're publishing in, but in my field the main author (the one who did all of the work) is the first author, followed by people who contributed data (middle authors) and the senior author is the person who directed the project/ran the lab/got funding. It's also necessary to specify who did how much work. As far as not including people on the author's list goes, if he/she had intellectual input and you don't include their names, it's plagiarism (which is very serious in academia). It's better to pad the author's list (soft authorship) and include a few more people than to plagiarise, IMHO.