Of course MS Works isn't supported by MS any more, but I've been using it since the DOS days; and I can assure you it uses only minimal resources. Since Kingsoft Office is like MS Office, I'd suspect it uses a good bit more resource. Another good free office suite is LibreOffice, but again it uses more resource than MS Works because it does more.
As to what's making his PC run slow, that depends on a lot of things requiring more info to answer. It might be that he just needs to get a more up to date computer, or maybe he has a lot of tasks being started. In the latter case he could run MSCONFIG, click the Startup tab, and uncheck some things. Anything there can be unchecked except the antivirus program.
Good luck.