Lightroom and WB on a Raw File

He is correct. A raw file does not have a white balance yet, and in the Basic panel that is where you set it. There is only one white balance. Anything else, like a local adjustment in a mask, or these sliders when you have selected an RGB file (jpeg, tiff) which already has the white balance applied to its pixels, is an adjustment of the colors, not a white balance setting.

As I understand it- WB is set in the Basic Panel with an indication of Kelvin degrees. This is a Global setting.

In the Masking panel you are only changing the 'Amount' -100+100 of the global set WB where it is not referrered to as WB but instead 'Color'..

Try an extreme WB in Basic panel, and then note that a Mask adjustment for 'Color' (NOT WB) is very limited.

So I guess your fellow photographer is basically correct 🙂