No, it’s not safe. If she is rear ended, the can may rupture and catch fire. Automobile gas tanks are fairly well designed to resist this kind of damage.
The best place to store emergency gas is in her car’s gas tank. Provided her fuel gauge works properly, all she needs to do is remember to refuel as soon as the gauge drops below one quarter. With a quarter tank, she should be able to go at least another 75 miles before running out. That’s enough to get her through any traffic jam. If she is paranoid about it, refuel when the gauge drops below half.
Something else to consider is that most (maybe all) modern fuel pumps are located in the gas tank and are driven by an electric motor. They are cooled by the gas in the tank. If the tank runs dry, there is no cooling and the motor will burn up. Replacing a ruined fuel pump will cost several hundred dollars.