Which SLP Plugin Style are they using? You can test their site by viewing their page with the locator on it and viewing in the console, you can inspect page using any browser tools and see if the theme is overriding any settings.
Assume since they have hired you for web design or maintainence, you will be able to find your way arounf the developer tools in any browser.
If you need to see how to do this please go to the SLP documentation site and research CSs or themes or plugin styles. They will need the Experience add-on if they need to make any changes to force their theme to comply
https://docs.storelocatorplus.com/