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GoreTex is already waterproof, but I guess you want to know how to make the outer fabric water repellent. Is it leather or a synthetic material?
For leather he can use Nikwax or similar products. For synthetic materials a spray-on treatment is better (don't really know what's available in North America since I'm from Europe).
But since the boots are new the outer fabric should repel water properly.

I've used Sno Seal for years and it's never failed me. I've never had a leak. Some will claim that Sno Seal will violate the "breathability" of the Gor Tex. But, Gor Tex is a joke, so don't worry about "breathability". IMHO.