Think of it as the difference between flinging a wad of sealing putty vs. a steel ball.
A jacketed plain round can bounce around inside the body at surprisingly extreme angle, then leave a small exit would going in any direction including back toward the shooter.
There are varying degrees of HP, some nothing but a lead cup (a handloader can make a wadcutter round with the slug reversed), some with a small soft cup surrounded by a (relatively) thick copper wall (which may be slotted so it peels back more easily.
Some HP will flatten & produce a blobby shape, others practically explode on initial impact. This will vary depending on a whole range of factors -- power transfer, flaws or dings on the slug, obliqueness to impact surface. And if it's fired through glass, sheetmetal, or even heavy clothing, the mushroom effect starts early, so there's a whole range of other factors.
You've gotten dubious information. HP will readily make a hole in a human torso, especially at that close a range. My feeling is .45 auto is one thing, but a .45 revolver would pack more power.
The only reason I'd advise you to study this a little is so that you tell the tale more naturally. In reality, there are so many factors that the range of outcomes is nearing vast. So long as you remain within those borders, all you need to do is tell the story well -- & nothing's stopping you from stretching from "likely" to "possibly" if you want to try.