TL;DR Get a sony.
I wouldn't get any kind of DSLR unless you're dedicated to pure photography. The video on them is necessarily clunky since the camera has to open the shutter flip the mirror up and pretend to be mirrorless while displaying the live preview on the back screen. Live view in general on DSLRs isn't a good experience. Everything on a mirrorless camera is live view. The viewfinder is a tiny OLED behind an eyepiece, the rear screen is another IPS or OLED. Sony lets you set whether you see the image as you'd see it on a DSLR (as close to pass-through as possible) or with settings applied. There's not much reason to use the first version but some people might prefer it. The viewfinder is high enough resolution you won't really miss an optical viewfinder.
Something to worry about as much as the brand is how much the lenses cost and how many third party lenses exist.
That's why usually I suggest that if you must go DSLR, get a Pentax. They're still making them and are pretty much dedicated to it (you can get either the regular or a monochrome-only version of their APS-C camera, and they make / made full frame and medium format digital too. Their medium format digital was surprisingly affordable and I nearly got it instead of the a7R IV... the lenses for medium format are the opposite of cheap though. There are multitudes of good lenses floating around for affordable prices used. Canon and Nikon are likely on the verge of killing their DSLR lines and their lenses tend to be ripoffs whether new or used. That will only get worse when they stop making them.
Sony makes the sensors for almost everything, and they get them first. More importantly they license their lens mount all over the place so you can get some amazing lenses for next to nothing in manual focus and far cheaper than in the DSLR era as autofocus. Sony themselves have some expensive lenses (the Grandmaster series) but most of theirs are competitive and can be had cheaper used.
Speaking of that, Sony's autofocus is unimaginably good. I'd always had people tell me Nikon had great autofocus, but I honestly thought it was pretty mediocre when I had the D800 for years. My APS-C Pentax K7 was massively faster to lock focus on the AF lens I had for it. Sony has human / animal eye AF modes on most cameras and other more advanced modes on some. With human eye you can normally just enable it and select left or righ eye for focus and it'll work. If your're shooting a group and want to make sure it stays on a person, you can program multiple people in by photographing them from a menu and assign them focus priorities. Animal eye AF doesn't let you pick an eye (on mine, newer ones might) but it locks on fast and tries to re-acquire focus even if the animal turns away for a second. The eye AF thing makes filming people far easier.
If he's mainly doing video and actually wants to work with video lenses, lots of companies make Cine lenses for Sony FE because they've used the mount on their insanely expensive movie cameras. They'll run you more though, and aren't necessarily any better, they're just designed how filmographers expect them to be.
I've done tons of photography with APS-C DSLRs for about 15 years, then switched to full frame. Full frame 35mm is better in general. Right now, Sony's two cheapest non-used full-frame options are essentially the same. The difference is that the first is normal sized with the usual external buttons and knobs (on Sonys you can customize nearly any button to do what you want, so if he's doing video he can set it up for that. The second, the 7C, is basically just a compact version of the first one. They stripped off some external controls and protrusions to make the body as small as possible. I have an Alpha 7R IV which is one of the high resolution series geared more towards studio and landscape photography. Any of them will do video, but some can do higher bit depths / bitrates than others. I personally wouldn't be able to deal with the compact model, the regular models are already very small for full frame cameras and I mostly shoot with either a Sigma 14-24 f/2.8 ART or the Sony 90mm f/2.8 IS Macro. Those are both sort of mid-priced. The Sigma is better than Nikon's full frame DSLR version I used when I had a D800 for a while IMO, and cost roughly $1300 less. I wasn't impressed by Nikon. My cheapest lens aside from adapted Pentax lenses in manual mode is the <$400 Samyang 45mm f/1.8 which is a nice fast lens and very lightweight if you don't feel like carrying much. I did a portrait shoot with it for a friend's daughter's HS graduation photos and everybody loved them which is all that matters since I don't do this for a living.
Alpha 7 mk III -
https://electronics.sony.com/imaging/interchangeable-lens-cameras/full-frame/p/ilce7m3k-b - $1600 ($400 off right now)
Those originally came out several years ago and are discounted since the second revision is out
Alpha 7C -
https://electronics.sony.com/imagin...s/all-interchangeable-lens-cameras/p/ilce7c-b - $1500 (sony is out of stock, can likely find it elsewhere and sony requires all distributors to honor their sales)
Note that Sony makes their flash hotshoes as a fully digital multi-pin slide in interface, and they can run all kinds of things other than flashes. Video lights, external microphones, etc. This might be pretty handy.
The big thing that would stop either of those cheapest ones from being an option (aside from wanting to spend less) would be if they want to be able to shoot 4:2:2 10-bit S-Log3 or HLG for grading to HDR. The other cameras can shoot 4:2:0 8-bit S-Log3 and HLG but you'll be stretching the data out to fit the full SMPTE2084 HDR10 space. The mk II version of the 7C is probably the cheapest you can get that'll do 4:2:2 in multiple formats including all-intra-frame for editing.
The other, far cheaper option, are the APS-C models they designed to be a out of the box setup for vlogging.
https://electronics.sony.com/imaging/interchangeable-lens-cameras/aps-c/p/ilczve10-w
The original version is heavily discounted from sony right now @ $600 and is a 24MP camera like the A7MkIII and A7C, just a 1.5x crop sensor. I like very wide angles too much to go back to APS-C, but that's a great price if it doesn't matter. Once again there's the choice of whether you need 10-bit video since this doesn't have it. The new version does have it but costs $1000. Nothing is ever free in photography. If I cared about video I'd have wanted 10-bit 4:2:2 because even without HDR there's more room for editing, but it would have still been low on my list.
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