One solution would be to hard code your changes in the theme's style.css file.
You can find the file following this path - wp-content > themes > yourtheme > style.css
Open this file using a file editor and you will be able to edit all the styling, including the font.
It will help if you use the source code as a reference so that you can find the correct classes and ids that are associated with the element.
It's pretty simple to change the font, you can use this as a reference to find the one you want - https://css-tricks.com/snippets/css/css-font-families/
EDIT: Here is an easier way that your client would perhaps understand if they have a very basic knowledge of CSS - https://www.inmotionhosting.com/support/edu/wordpress/change-wordpress-appearance/change-font-globally
Alternatively, there are a ton of other plugins that will do the same thing as the Google Fonts plugin. For example: https://wordpress.org/plugins/use-any-font/
Hopefully, this will provide you with enough information to help.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…