"18" means 18% reflectance not an RGB reading of 18, 18, 18. I know Lightroom uses R, G, B percent readout, but Lightroom also makes auto ranging adjustments to your exposure and tone curve, so Lightroom's color readouts are pretty much useless. In this case, you are using your gray target not just for setting a neutral white balance, but also for setting exposure.
If your gray card is really 18% reflectance, then this might correspond to middle gray in an RGB color system of around low to mid 100's in the RGB readout in your editor (R, G, B = 118, 118, 118 to about 126, 126, 126 or so). That number will depend upon the working color space you are using.
If you do not know what all of this means, just click on the gray card with a WB dropper or sampler and don't worry about it. Just slide the sliders and read up on the subject when you have the time. You might also go to the website of the gray target manufacturer (Lastolite or who ever made that target) and see if they have a tutorial that explains how to use their target. It really is almost irrelevant in the grand scheme of things.
Or ask your friend to elaborate. The problem with using white paper and all that is that some things that are "white" contain optical brightening agents that make them look bright white (but actually make them blu-ish), so your white balance off of them becomes warmer than neutral, which, again in the grand scheme of things, is probably fine.
kirk
