Take it to a gun store, not to Dicks Sporting Goods. Thats an ok place to buy stuff but not for serious help.
As already posted if it says 22short or 22LR then use those. A 22 short is a very low power round and would minimize any risk. IMO the chances of it being dangerous to shoot are quite low. The amount of powder in a 22 is very small and unlikely to cause a failure of the bolt or chamber.
Make very sure the inside of the barrel is clear of obstructions and rust.
Probably the biggest risk is a misfire due to a malfunction of the firing pin. Possibly cause a discharge at an unexpected time.
Take it to a real gun store if you’re not comfortable. Remember 22s are quite capable of killing a person.
He did not say Long rifle, he said long/short. Longs are different in power than a long rifle and should not be fired in a long chambered fire arm. Most .22s of that era were chambered for all three of the most common rounds but some were not. He needs longs or shorts. Longs are almost non existent now so he will probably have to make do with shorts.