WordPress - user access levels

I’ve been asked to create a site for a friend to promote his business and was thinking of using something like BeTheme, Avada or X to do so - he’s a freelance horse trainer who basically goes to wherever the client is. One of the things we’d like to have on the site is some kind of calendar section, whereby he or his PA could log in to the site and update the calendar with his availability and location on various dates.

Does this sort of thing sound feasible - either out of the box with one of the themes mentioned or with the use of an additional plug-in? More importantly, I don’t want him or his PA to be able to modify the look and feel of the site in any way - I know WordPress can have different levels of user with different access levels, but are they flexible enough that I could allow them access to some parts of the site, for example the aforementioned calendar, and not others, especially in the context of a site created using BeTheme, Avada or X?