Any caliber mentioned already will be sufficient. I've been thru a number of chamberings lately and keep coming back to common, readily available and mild recoil. A 6.5 isn't the best option if you want your wife taking 800yd and longer shots. Less than 500 it'll do fine, as will a 270, 284 or 308. She really doesn't need a magnum and likely is trusting you to pick a cartridge she can shoot comfortably without flinching.
Personally, I'd spend more time thinking about the projectile and matching it to how she plans to hunt. Does she need bonded lead, cup and core, all copper, frangible, etc. They all have their place. If concerned about penetration and weight retention it's hard to beat Barnes ttsx, tsx or lrx for off the shelf. But I've had great results with 270 accubond and ballistic tips too. They're not all drt but I haven't had to go more than 50 yards for them.
Fwiw, bullets don't typically take them off their feet when paced in soft tissue like lungs. Solid bone hits do sometimes but man that's a mess. Lots of wasted meat. If you want real knock down power, use your truck.
Below in the photo is nothing special. My cow from October with a couple of Barnes ttsx in a 284 just passing thru. Did I need to shoot twice? Not really. But if they're still standing I'll put another in them for a quicker kill. The bullets did a ton of damage inside before exiting. I don't take head, neck or spine shots, do heart and lungs are it for me. A lung shot takes time to expire. Heart, great vessels or aorta is quicker but a smaller target and sometimes hiding behind the trailing leg. Shooting thru bone, guts or thick muscle can slow down a bullet. That's where more mass, momentum and weight retention can help. For those shots a magnum has an advantage but I'd sure hate to rely on that and take shoulder shots. Just keep them in the boiler room with a quality projectile and get your game bags filled.
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