[asshat]

You forgot to tell the boy to undermine the existing patio slab so it doesn't heave.

This would probably be a good idea, and drilling into the existing concrete would also be suggested. I didn't want to over complicate it.

The ideal, would be to dig a small beam under the existing concrete around the perimeter of the hole. About 6 inches deep and 6 inches under the existing edge. This will lock the two slabs together and keep the new concrete from heaving up above the existing slab in case you get any ground swell. Also, over dig the hole about 2 inches and use some cushion sand underneath. This will also minimize the heaving.

In addition, get a small hammer drill and drill some 1/2 inch holes into the existing concrete midway. Drill them 6" to 8" into the concrete. Take about 18" long pieces of #3 rebar and dip them in a two part epoxy and stab them into the holes. This will keep the slab from settling and the joint from pulling apart. If you are concerned with any expansion problems, you should use a 1/2" asphalt expansion board around at least two sides.