I would go with a .22 if I was him. It is still far cheaper than Milsurp, and you don't have to clean for corrosive ammunition. And my Mosin beats the crap out of me. Your friend just doesn't sound like he has the motivation to care for a gun, much less a milsurp. If he gets one, it sounds like he will shoot it a few times, then let it sit leaned against a wall or under the bed while the salts destroy the barrel. I love my Mosin, but even I don't shoot it nearly as much as the .22 as there is much more hassle involved in cleaning, etc, and the .22 is more fun. You have got to be motivated to shoot and care for milsurps.
A .22 is a lot easier on the ears, shoulder, and wallet than a full powered rifle. He needs to learn the basics, and a .22 is good for that. 6 clips (6x5=30 rounds fired) does not make one a marksman. It merely gives them a taste. The cheapest surplus, which I am guessing is 7.62x54R, is around 10 cents a round online. In reality, at the gunstore, I pay the same as you would for the 8mm ammunition. .22 bulk pack costs about 1-2 cents per round, and in some cases, less.
Regardless, it sounds like he will not be able to get one until he is 18, or his parents get one for him. Ammo prices will change, and 8mm surplus may be a lot harder to find. He also needs to take a basic safety course.