The gasline runs along the bottom of the car body from the gas tank to the engine. It is usually routed along one or more frame members to protect it from possible debris damage. Usually gas lines are made from steel or from rubber with a braided steel covering.
Usually, a gas line won't freeze. If that is actually the problem, then your boyfriend has water in his gas tank and fuel system. If he doesn't keep the tank full in cold weather, water vapor from the outside air can condense in the gas tank and get into the fuel system. To repair the damage, once you get the gas line thawed, run the maximum amount of fuel system drier through the car. I use HEET and put in a bottle every time I fill up in the winter. The water in the system will also destroy the fuel filter, so he will want to replace that also. Unless you have very cold temperatures, -15 or below, the problem might be a clogged fuel filter or a clog in the fuel line. I am assuming that the battery is ok, and he has checked all the other possibilities, of course.
To thaw the gas line, I suggest using a hair drier or a heat gun on low, since a torch can get too hot and cause damage to the line or the coating on the underbody. It will take a long time, and unless there is something to keep the line from re-freezing, it might be a futile effort. I suggest moving the car into a garage or pushing it into the sun to allow the temperature to come up a little.