Early, in the worst shoot-out the FBI ever had, an Agent shot a Creep in the ribcage with a 9mm from a short distance. The bullet, while on the correct target path and with sufficient velocity, deformed and stopped before the bullet traveled thru to the Creep's vitals. Said Creep continued his attack despite soaking-up his new less than lethal gunshot wound to his chest.
The official FBI shooting review concluded that had 9mm been loaded with a bullet capable of decent penetration the Creep most likely would not have had the energy to kill, and injure, multiple Agents, shortly after the intial hit.
Reflecting on the tone from your request for this 25 year old open-source information, I am certain that others, whom may know more about the Dade County Shooting than me, will gladly fill in the details if you so desire.
Here's some details on that shooting, the 9mm bullet (147gr Slivertip, I believe, but would have to look that up) struck the torso from the side, and stopped at the top of the heart. It took out the aorta. IT was a lethal wound. The ME at the time said, "if he had taken that shot at the operating room door, it would be problematical if they could save him" (paraphrase, its been a long time...
That bullet did not fail. It was a lethal wound. What it did not do was instantly STOP the creep. He lived for a bit more than a minute, and during that time fired over 100 rnds from a mini14, killing and wounding several agents, before finally being stopped by hits from a .357, fired at close by a wounded agent as the creep was trying to drive away.
What failed in that shootout were the tactics, training, and shooting of the agents involved. What they were up against was something outside of their experience and training. The creep was not just a bank robber, he and his partner were stone cold killers, who fired hundreds, possibly thousands of rounds every week for practice! They murdered several people plinking in the swamps, to get their cars, and guns. They did stop those creeps, but at a huge cost.
The FBI, however, decided the official failure was the bullet. And so began the process that lead to adoption of the 10mm (which was just too much), the 10mm Lite load (which met the desired specs, but was still in 10mm guns, where were "too big") and finally the .40 S&W (delivering 10mm Lite performance, in a smaller, more easily managed gun platform).
As to the .38 wadcutter, and particularly the .38WC loaded light, it is not a good choice for self defense. A friend and I loaded and shot a few thousands of them (from a 6" M19) back in the early 80s.
Loaded them backwards, and right side up. HB and BB both. He liked to load them at about 600fps. Nice load, virtually zero recoil, and quite accurate.
Penetration? not so much. And, while its a nice medium for general comparisons, don't put too much faith in ballistic gelatin. Real world target: junk car, mid 60s Galaxie. Range 15yds-ish. My friend, a fine shot, could pick the F O R D letters off the trunk DA, faster than I can type this. The slugs usually went through the trunk lid. Usually.
Target: abandoned refridgerator. .38 HBWC upside down. Vel: ~600fps. Range: ~10yds. Several cylinders fired, maybe half a dozen slugs penetrated the outside of the fridge...most left largish dents in the metal, opened up to the size of a quarter, flattened out well, and were picked up off the ground at the base of the fridge.
These round might get in to the vitals of an attacker, if there was nothing more than a t-shirt in the way, providing they didn't hit a bone. Maybe.
The combination of the flat wadcutter shape, the soft lead, and the low speed means they will NOT penetrate deeply in flesh, and do NOT deliver a large amount of energy (of which the .38 doesn't have much, even in full power loads).
They mush out real purty...that looks impressive, but the performance just isn't up to what I (and most others) consider a reasonable standard for self defense.
Better than bare hands, certainly, but not much better than the pointy stick, except you have a little stand off distance.
A soft lead WC, at the low speed of ~600fps is a fun plinker, and cuts nice clean holes in paper. It also can be stopped/deflected by nearly any hard object. A belt buckle, metal studs/chains on a leather jacket, even just the arm of the bad guy, if that light.38WC has to go through any of those, or something like it, odds are its not going to get where it needs to go to stop an attacker. Not a good choice, in my book.
Now, a hard lead SWC, driven at high speed, is a completely different thing. Lots of penetration there!
The light .38 "poppers" just aren't "stoppers"!
If that's all your girlfriend can manage, its still better than nothing. Her just having a gun (and the confidence to use it, if needed) might be more than enough. Statistically, just presenting the piece ends more fights than shooting does.
However, you can't count on that, either. If the girlfriend can't manage anything but poppers, so be it. But, if she can handle heavier loads, and just doesn't like them, that's different. Let her shoot all the poppers she wants for play, and see that she can do a cylinder of "stoppers" once in a while, "just in case, hun!" If you approach it the right way, both of you might be surprised with the eventual result.