That's a pretty hefty cut height for shooting with, it's not the size of a clothes peg.Point is, he should be easily able to use a 4 to shave the end of that bit of wood in the pic. If he can't, it probably doesn't matter what plane he uses. There's something fundamentally wrong
If really suggesting to use the smoother, then try on something that size.
If the piece were twice that thickness, the work is difficult no matter what plane used.
about the limit for my 5 1/2 and anything beyond will not get planed the same way on the board, (shearing with a short plane using the board as a stop instead)
As I said sometimes the wood doesn't like getting cut from one side.
If you try turning the piece so you're planing that same endgrain from the other direction it will do two things.
Making sure the piece is parallel, and making sure that the square is accurate...
should the square for checking the shooting board be accurate both inside and outside!.
The more accurate the finer the cut you can take, which might be what the issue is,
i.e not getting a productive cut, and then slamming into the wood with too deep of a cut.
(which does the edge in on my timbers)