It depends on the browser they are using.
If they are using Internet Explorer then the simplest way for them to choose to make the current page their home page is to go into the "Tools" menu and select "Internet Options" then at the top of the "General" tab press the "Use Current" button followed by the "OK" button.
Of course if they already have one or more home pages set and want to add the current page as an additional home page then they will need to copy the address from the address bar and add it as a new line in the text area above the "Use Current" button and then just press the "OK" button.
It will be similar in other browsers but the exact names of the menu options and buttons to press will vary.
Setting home pages became a lot more complicated since tabbed browsers were created since now you can have as many home pages as you like all at the same time.