The photos show both a left side (non drive side for single bikes) crank arm and pedal. My first thought is that the pedal wasn't installed with grease on its threads, resulting in galvanic corrosion. I see what looks to be rust on the steel axle suggesting enough time exposed to water, add road salt and time and you have a stuck seatpost, I mean pedal.

My second thought is that the bike was ridden with the pedal not fully tightened in the arm and the loose pedal axle acted like an auger to the soft arm.

Either way a possible fix is to install a Helicoil into the arm and make sure the pedal threads are clean, undamaged, lubed before installing it. Is there a LBS near you? Some shops won't do this type of repair, it is a method with varying results and is dependent of the skills of the person doing the fix. Having a pedal fall off the crank arm when riding is not something a shop wants to deal with (and hopefully their liability insurance is paid and is large enough to cover the possible rider injuries. The far more "reliable" fix is to replace the arm. Andy.