The tool to install that the UX Designer needs is the Sketch plugin. This plugin allows the designer to quickly mock up Salesforce user interface experiences using a collection of prebuilt components, such as base components, tokens, design patterns, and more. The plugin also provides access to the Salesforce Lightning resources for their design and prototype, such as icons, fonts, colors, and images. The plugin is compatible with Sketch, a popular design tool for creating user interfaces.