We have an established side match (Wild Bunch) which utilizes the Mexican Punitive Expidition timeline, in which 1911s and Mausers (03 Springfields, or 98s for the Villistas) can be used. For the main Match though, No. THey can come watch, or use Scout guns until they're ready to try our unique weapons.
Remember, when SASS started, Single Action Revolvers were considered a dead item. But if you wanted to play Cowboy, you had to get a real Single Action, not just simplly thumb cock a double action.
You want to play GAF Battle Rifle, you need the right Victorian era weapon.
And consider this: We require speciaty ammo for these rifles. Your gonna ask him to take the time and materials to load up GAF spec loads for his ineligible weapons, only never to do it again? Best be up front, do it right or don't waste the time and money.
I'd ask him to go to our website and read our regulations and listed weapons. He'll either understand, or he won't. Its not for everybody.