It would have to penetrate the intracostal muscle, the lung, and then the pericardium to injure the heart. I don’t think a pellet rifle could do that. The most powerful pellet rifles I’ve sold had 29 foot pounds of energy at the muzzle. Those were .25 caliber Beeman R-9s, very unusual guns. I doubt this guy had one. A .22 pellet has around 15 foot pounds at the muzzle. By contrast, a .22 CB Short has 32 foot pounds.
The pellets might weigh as little as 7 or 15 grains. The .22 Short bullet weighs 29 grains. Mostly these things can’t be expected to penetrate deep into a limb. The muscle between the ribs should have stopped the pellet. As I think about this, I wonder if the woman had a medical condition that was a factor in the accident. It could have entered the lung and lacerated the pulmonaray artery, but it seems almost impossible that it was a fatal injury without some other factor.
i was thinking something really vascular in the back.. like the kidney or lucky/unlucky angled back shot nicking aorta/vena cava or other major artery.