A 10/22 semi auto with high cap mag if legal in your area firing high speed hollow points would be a better choices as a cheap indoor defensive weapon than an SKS. A 22 is relatively weak but deadly when used with skill and effective agains an unarmored BG. Also a 22 is a cheap weapon with which to become proficient. Large pigs and cows are dropped dead every day with a high speed .22 LR cartridge. A stinger round would have the least penetration and most likely only make an entrace wound and not an exit with roughly the power from a rifle as a 380.
A pistol caliber carbine if one wants a long gun would be a bit more effective choice than a 22. A 357 lever loaded with 38 hollow points or a 44 mag carbine loaded with 44 specials would be relatively quiet and about as effective as a .357 from a short barrel. Ruger had a semi auto PC9 and PC4. I recommend them highly if you find them. Marlin had a camp 9 and 45 that are good but not as reliable.
If your friend won't shoot much to become proficient a shotgun is a decisive fight stopper inside a home for self defense. It is however loud, but less so than a rifle round. 410 with buckshot, 20 guage with bird or buck, 12 guage with bird or buck will all work wonders on changing the attitude of an attacker. The larger two guages will probably end a fight with one blast of buckshot.
The most powerful semi auto I would even consider for an idoor defensive weapon is a 30 carbine shooting hollow soft points. The only one currently avaialable is a the KAHR arms reproduction and the only load that is a hollow point is made by Winchester. These also cost $700+ and are not the ideal defensive weapon.
22, 20 guage, and 12 guage ammo is everywhere and weapons are relatively cheap.
9mm, 38, 357 and 45 auto are about as common as dirt.
A home defense gun is idealy a powerful short range weapon and not one of intermediate or long range due to the danger to bystanders.
A 7.62x39 round is just too powerful and will zip through your assailant and the wall behind him and potentially several houses depending on the structures encountered such as particle board and sheet rock rather than 2x4s, water heaters, bricks, cinder blocks etc.
Responsible gun ownership means behaving responsibly with guns. I believe using a 7.62x39 for home defense in an urban setting could be construed as reckless disregard for the life and property of others should you hit an innocent person. You also could be villified in court as having used a particularly deadly "assault rifle" because the media and average person doesn't really know the difference between assualt rifle and an SKS. rc