No such thing! He'll have a terrible smelly, useless, foul tasting mess.
The anchovies have to be digested by their own digestive enzymes.
Easiest way to have garum is to go out and buy a bottle of Thai or Viet Fish Sauce. The Romans had both clear and murky versions, and so do the Thais, Vietnamese, and Filipinos.
In most cases, the murky is a byproduct of making the clear, but in the Philippines, the clear is the byproduct, thus lesser quality than Thai or Vietnamese.