First off, the .45 Colt and .410 bore shotgun are, indeed, the same diameter. Remember, in the US ammunition is named based on what is going to sell, not any number that makes sense. Look at the .38 Special -- it's a 36 caliber, nominally sized at .357, not even close to being a 38 caliber round.
So anyway, they are the same size, and the Judge is designed with chambers to fire both rounds.
So, essentially, what your friend had done is load his own .410 slugs. He hasn't made anything new, just short slug loads.