Is this the Yukon XL? I am not familiar with the regular Yukon (same as the Tahoe) but I have a 2000 Suburban, which is similar to the Yukon XL. And yes, it has forward of the bight seat belts in the 3rd row, which are really annoying.
I can get my CA65LX installed securely in the center 3rd row position by reclining it (it has two allowable angles for forward facing) and pulling it away from the seat bight a couple of inches. When I install, I get in the seat and push down with all my weight to tighten. Not back-and-down like I would if the belts were coming from the seat bight. You want to maintain a gap between the back/bottom of the car seat and the seat bight, so the belt path stays lined up with the seat belt. It takes a little (or a lot) more force to get it tight this way, but once it's in it's solid and I don't think her son will be able to loosen it.
This trick works with most seats that have a recline mode, but I don't know how well it will work on a CA if it's not the LX with the reclining base.