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Oh, now that is absolutely precious! . When something like that happens, isn't is super-special!
I hope you wait until he is in a public place with other avid photographers present, and then bring it up and force him to eat crow and have it be evident to all how very wrong he was!
Opportunities to show someone to be so very wrong in such a thorough way don't come along often. . I hope you're able to make the very most of it!
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He's rather special 
Humans and hardware are not perfect. That is a reality!
There is a difference between mirrored and mirrorless. When I use my sony and when it FAILS I go into manual focus mode and use focus peaking to "attempt" to land the shot during an event. This is what happens I fully admit the weakness. I do find difficulty when some claim zero issues and then start to point at "me" the person behind the camera lacking knowledge in usage >>>>> If I used my 5dmk4 with Flash and utilizing the red focus assist (one shot AF-S mode) I almost NEVER get a sinking feeling of missing the shot. Bounced flash stop motion blur and I can crisp sharp images NEVER missing a shot due to sluggish slower AF shutter or AF hunt like mirrorless.
Now if I'm shooting no flash, available light all camera's can potentially have issues or difficulty if the light is low. If you use mirrorless with flash the AF-S and LED onboard camera can sorta work. Godox is pretty much the only external 3rd party flash capable of emitting a red focus beam. NOT reliable and the mirrorless camera can still hunt or be slow on the af "lock".
I know many that will NEVER admit this on public forums....they have to uphold a flawless reputation......gag.......
Pros and cons between mirrored and mirrorless.
???? That is not cool IMO. This is where a mirrored body would have zero concerns in doing the job. 



