I took a class from Jamie Wallen, and he said that a meander works best when basting a quilt. He says it's better than a grid at keeping everything flat and square. A lot of quilters baste in a grid, though.
Since your friend plans to machine quilt, I would strongly recommend using water-soluble thread for the basting. Superior has two good ones, and I would use the heavier one in both top and bobbin for basting.
Thread basting is fine for hand quilting, as you clip and remove basting threads a section at a time. It is usually not used for machine quilting (unless you baste with water-soluble thread) because it is difficult to remove both the top and bobbin threads for the area you want to quilt.
The Superior water-soluble thread is on the pricey side, but worth it.
Edit: Maybe the two of you should glue-baste or spray-baste the quilts instead?