The salt era guns were produced from 1969 - 1972.
They used salt to speed up the drying of the walnut.
The salt was thought to leach off during the drying process. It didn't and guns began ro rust. Attached is a good explanation.
I had the opportunity to shoot this gun last weekend. the owner is not a shotgun shooter, the gun was given to him by the wife of a friend. I will get serial numbers to get the date of manufacture. It was fun to shoot
Most long tang guns were made before the salt wood was used for stocks and forends. So it's doubtful that your friends gun has salt wood. If the problem hasn't appeared by now (50 years later), the gun is safe.
Spitter I've read that also. However, I have three salt wood Supers that are all Grade 1. The guns were repaired, re-blued, and new wood installed. One would never know their history unless the stock or forearm is removed. And they all shoot reliably. Unless you're a collector, properly restored salt guns are as good as non-salt guns.