I told him, since the invention of “raw editors” like Lightroom and camera raw in photoshop, you can certainly open raw files, the software simply shows a visual representation of the raw data and makes local copies.
Raw editing/conversion applications do give A representation of the data in a raw file on your screen. But we must be careful to not think they give THE representation of the data in a raw file on your screen. They don't.
In that respect, your father is correct. The data in a raw file must be processed to present anything remotely resembling a viewable image. How that data is processed and presented on screen varies greatly from one application to another, and even within the same application depending on what default rendering settings are selected when the raw image file is first opened. That's why opening the same raw file using different applications can give radically different results.
Raw image files contain enough data to create a near infinite number of interpretations of that data that will fit in an 8-bit jpeg file.¹ Anytime you open a raw file and look at it on your screen, you are not viewing "THE raw file." You are viewing one among countless possible interpretations of the data in the raw file. The raw data itself contains a single (monochrome) brightness value measure by each pixel well. With Bayer masked camera sensors (the vast majority of color digital cameras use Bayer filters) each pixel well has a color filter in front of it that is either red, green, or blue.² For a more complete discussion of how we get color information out of the single brightness values measured at each pixel well, please see RAW files store 3 colors per pixel, or only one?
How the image you see on your monitor when you open a raw file will look is determined by how the application you used to open the file chose to interpret the raw data in the file to produce a viewable image. Each application has its own set of default parameters that determine how the raw data is processed. Since each application uses a different set of instructions to process the raw information contained in the file, each result will be different.
One of the most significant parameters is how the white balance that is used to convert the raw data is selected. Most applications have many different sets of parameters that can be selected by the user, who is then free to alter individual settings within the set of instructions used to initially interpret the data in the raw file.
¹ Sure, you could take a picture that contains a single pure color within the entire field of view. But most photos contain a wide variation of hues, tints, and brightness levels.
² Except the "red" filter is really more of a yellow-orange color, the "green" filter is more a yellowish-green color, and the "blue" filter is a violet-tinted blue color. In other words, the colors of the filters in a Bayer mask do not correspond to the three colors our RGB monitors emit and blend to reproduce the response in our retinas that many other colors do. In fact, the colors of the filters in a Bayer mask are much closer to the three colors that each of the three types of cones in our retinas are most sensitive to than they are to the three "primary" colors we use for our RGB color reproduction systems.