If it was made that way before 1990, it is legal. If not, it may violate* 922(r). If it was made prior to 1998, the Statute of limitations for the 922(r) violation has run out and noone can be prosecuted under 922(r). But, it may still be considered Contraband.
If it was made after 9/14/1994, it is in violation of the 1994 AW Ban and is illegal for a civilian to possess.
*The US Folding Stock, US Pistol Grip, and US Magazine make 6 US Parts (Handagurds, Stock, Pistol Grip, Magazine Body, Magazine Follower, and Magazine Floorplate). Assuming that he has enough US Parts (6 or 7 on the SKS. Can't remember which) in it, it could have been made between 1990 and 1994 and not violate 922(r).
If it was made after 9/13/1994 and isn't marked GOVT/LE ONLY...Then it is illegal.