So I was planning on adding 3/4" grosgrain loops to my 12'x12' ripstop nylon tarp to give myself som better ridgeline and tieout options. My wife is going to use her sewing machine to sew them where I locate them for her. My question is: What is the best way to waterproof the area that she sews them onto?

I'd think that the tarp is waterproof and all you'd need to do is use the appropriate seam sealer on the sewing. Might be a good idea to find some seam tape for her to sew in too.

Might consider asking in the DIY subforum, but boonie pretty much nailed it. Seam-sealer is also great to strengthen seams/sewing. Just need to know if your fabric is sil or urethane coated.
Both types of sealer are readily available for this purpose. Both also have analog counterparts from more general applications, like construction, for less $/oz.

Just buy some Gearaid seam sealer.

make sure you reinforce the loops with a fabric patch. Ripstop nylon or nylon fabric works well. Loops by themselves tend to tear

I just finished making my 3rd tarp yesterday. All of mine are 1.1 oz silnylon. I am far from a professional (as my crooked stitches prove), but two of my tarps have successfully weathered days of sustained strong winds and storms and don’t yet show any stress.

I do suggest as others have mentioned that you have her provide some additional backing if you are just sewing them to a rolled hem or seam without any reinforcement. I think this is especially important if adding loops to the interior section of the tarp. I use a double layer of the same material I made the tarp from. For sides, I get a plastic lid and draw a circle in a scrap, then cut it out and fold it over, stitch the edges so it stays folded, then sew it on the spot I want the reinforcement. After that I add the loop. In the center, I cut a rectangle, fold it into a square, fold the edge 1/4 inch, then stitch that down to protect the edges and hold it in shape while I stitch it onto place. Alternatively, instead of backing, you could just add a piece of grosgrain along the edge of you tarp and sew the loops to that. It would help spread out the stress from right at the loop, but I have not done it this way so can’t speak from experience.

As mentioned above, your tarp fabric should already be waterproof but all those new stitches create little holes and the threads could wick moister in. It probably won’t drip on you, but I still like to seal up the top with regular seam sealer. In the photos below, you can see the reinforcements and loops on my new yellow/green tarp have not been sealed yet (waiting for nice weather to do it outside), but on the grey tarp you can see the shine which is the sealer.

Also, ask you wife to avoid cotton thread. Polyester is much better for outdoor gear.