Maybe he's doing something like ceremonies, weddings, events, etc, and needs cameras with common batteries, lenses, flash triggers, etc.
Example, I want the Tamron 35-150 SO BAD; I could hold onto my R6, and ditch my backup body (Eos R) to buy a A7III to use the Tamron with, while I'll use my bright primes (40 Art and 105 Art) alternatively on the R6.
I buy the A7III with the R money (they cost the same used), but to buy the 35-150, I need to sell the 24-70 and 70-200. Switch must be zero charge.
Now, if the R6 dies in the field, I can use all the lenses (included the Sigma DSLR primes via MC-11 adapter) on the Sony; but if the Sony dies? Then I find myself with two beautiful primes on the R6, but certainly not that flexible, and the 35-150 zoom it's useless (and I already sold my two Canon zooms to buy the Tamron), and nothing wider then 40mm, so not an ideal situation (even if you certainly can do a wedding just with a 40 and a 105 if needed).
So, it's not so uncommon to think "well, the moment I buy a Sony alternative, then I'll just buy two of them and ditch both R and R6 altogether"; I thought that already, but haven't pulled the trigger, yet. But maybe he has different needs, and urges, then me.
Yes, many people have different brands to do different things; but if you need to do just one thing, or different things but with need of backup, then you (usually) just want one single brand, not two or three.