Makarovs and 8-round mags are perfectly legal.
When traveling, the gun needs to be unloaded in a locked container (loaded mags outside of the gun are fine and have no storage/carry restrictions). Do not tell cops about the gun nor give any indication of its presence. CA is not a duty to inform state. At best, nothing good can come from cops knowing about guns (especially in more heavily Democratic or urban areas). If transporting it on foot, he must be going immediately to or from an authorized location (like from his car to hotel, or going to a shooting range, or whatnot).
He can have it loaded, carried openly or concealed, within his residence (including a campsite, RV, or under some circumstances, other vehicles, when adapted to such use, such as a pickup with a tent in the bed), or on enclosed private property not open to the public if it is not prohibited by the owner. He can also loaded open carry in unincorporated areas where discharge is lawful (which excludes all official roads, so no such carry in a vehicle unless off-road in such an area).
Obviously, any CCW permit he may have from elsewhere is not recognized here, and visitors are ineligible for non-resident permits (not that those are very common to begin with among the qualified).
Defensive laws are decent for a Democratic state, but the response to a defensive use in practice can vary by jurisdiction, with more Left-leaning or densely-populated areas tending to react more poorly to such things. He should do some research on our UOF laws.
Illegal carry, or illegally having a loaded firearm, can be potentially charged as a felony. Getting a gun back as a non-resident from LE after a lawful shooting or for any other reason is unlikely without some sort of lawsuit. The gun release process here is outrageous, and among other things, requires the gun to have been registered, which is impossible for a non-resident. Of course, better to lose a gun than his life or limb. A Makarov shouldn't be a huge loss, though.
There's no reason not to bring it as long as he follows the laws on carry, storage, transportation, and use of force, and can avoid telling the cops about it.