My friend got the units level by holding them in place then marking through the holes where hooks go. Then drilling the holes for the brackets to go there. Is that the best way to get them level?
How many hands has he got?
no need to balance the cabinets on your head while marking the wall. They might move a bit (or you might drop them)
Determine the finished distance from the ceiling to the hanger.
mark (at least) one hole position.
Climb on your steps
Use your longest spirit level to draw a line all along the wall and sensibility-check it to the ceiling (the ceiling is probably nearly, but not perfectly, level). Turn the spirit level round, left to right, and check it again.
If you follow my recommendation to use hanging rails, you can drill one hole at the end of the rail, and screw that, then hold the rail level and pencil the rest of the hole positions. You need fewer hands if you mark, drill and screw the two ends, then the rail will hold itself up while you mark the rest.
Having established where the first hook needs to go, you can measure the distance between them, on the back of the cabinet, to determine where the other goes.
Though if you use rail you can move them sideways as much or as little as you like so it doesn't matter.