You need to find a way to keep temps above 50F, a salamander is a bit overkill
A big one might be overkill. But a smaller one might not be. We don't know the size of the garage. Plus, depending on what he intends to use the garage, the heater might be nice to have anyways. I've done many woodworking projects in heated garages in the middle of winter. And I loth having to do a significant auto repair in the middle of winter in a cold garage.