Except that the "hose" of an SSD is 4-20 times "bigger" than the best he will get with an external mechanical drive, so again your suggestion is completely wrong.
Accessing footage from the SSD might once in a while get slowed a tad from the minimum 500MB/s it is capable of if a ton of other stuff is doing accesses simultaneously. Accessing footage from an external drive caps him at 130MB/s at BEST, not even talking of the hugely worse access times.
Using an external mechanical drive when he's got an SSD is the most stupid suggestion you could do...
On the contrary while you might want your long term storage to be on a mechanical drive for obvious cost/size reasons you're always better off store the material to the SSD until you're done working with it.
Not that having the footage on a mechanical drive would be much of a problem unless you're doing multicam/lowcompression editing, but it will never be "better".