I still say use death row inmates. The US has the largest prison population in the world.
According to my friend, that wouldn't supply enough of a sample size for medical trials, and it makes sense. Even if it somehow worked in the U.S. (ethical concerns aside), that would leave the rest of the world, including countries with fundamentally corrupt judicial systems that convict a lot of innocent people (e.g., Iran and Saudi Arabia), to figure out how to replace animals for medical testing. Imagine if Iran or Saudi Arabia tested on death-row prisoners who were on death row for "crimes" such as blasphemy or political activism.
Also, in my opinion, that would imply death-row prisoners were less human than the rest of the population, since you'd be violating their bodily autonomy just because they were prisoners. I'm strongly against that notion; dehumanizing prisoners or taking liberty with their rights just because they're prisoners has historically caused a lot of preventable death, torture, and suffering.