I will admit up front that though I've been a hardcore hobbyist shooter for more than a quarter century, I'm a novice with optics... though I've had the pleasure of owning a few and using many that have covered the full spectrum of "royal crap" to "WOW, this is like pleasure for my eyeball..."
Two things occur to me straight away when it comes to cheap optics.
First thing is that cheap optics give you really, really cheap glass and what you may not notice if you haven't played with the good and the bad out there is that your eye ends up having to work
much harder to see, especially when the zoom is cranked up higher. If you use a really cheap scope but you keep the zoom cranked way down, they are far easier to use before your eye simply says "I can't deal with this anymore", and for those who are prone to it, some manner of a headache can be involved.
When you spend more money on higher quality glass, you end up with a view that your eye can deal with far better and you won't get as "tired" trying to look through it.
The second thing is that you have a much,
much better chance at making a cheap optic actually work "acceptably" well if it ends up mounted on a rig that doesn't physically abuse it as much. You can spend less on a scope for a rimfire and take the whole "recoil shock" out of the the equation.
If your pal is going to mount a $79 scope on a .308 and you have done your due diligence to explain how bad an idea this is... IMO, this is where you step back and smile and watch all the absolute
WASTE you will witness:
--wasted money on that glass
--wasted money in his ammo
--serious wasted time and effort if it's handloads
--totally wasted time & effort in the buying, waiting for shipping, mounting
--more wasted time & effort at the range
...all for him to end up with a rifle that won't shoot because of his wild idea that a $79 scope is the answer.
Try one last "Hail Mary" on him.
Ask him to remove the emotions and simply try to look at it from a business standpoint.
Ask him how it's even possible that Leupold, Nikon, Zeiss, Swarovski, Burris and all of the high dollar rifle scope builders could ever possibly survive in this business when they simply don't sell
any rifle scopes under three hundred bucks, and they sell so many over $600... in a world where anyone with a computer can get a "similar" product for $79 shipped?