Probably not, if it's onions
Onions are a soft product so the damage from chopping this (weird) way is much more affected by what kind of cutting surface he's using rather than the product itself.
This is because the greatest impact pressure on the knife will be the point where it hits the hard surface of the board.
If you're worried about knife damage, just make sure he:
* Uses a softer cutting board (those cheap, thick white plastic boards are pretty good for this)
* Doesn't try to do this with products harder than onions, like coconuts, bone, or his own fingers...