Scorpions outside? Try closing the door! See? Wasn't that easy?

More seriously, I had heard that standard repellents do not work on scorpions. Gal of my acquaintance had to sell her home; it was apparently built on a nest of these critters. She would find them all over her home like roaches, in profusion. Doesn't anything out there prey on them to keep their numbers in line? Another reason not to keep pets, so naturally curious.
Kaaskop49
Shield #5103
Unfortunately, any slight failure in a door seal is an open gateway to a scorpion. I rate them ahead of roaches when it comes to getting into places they are not wanted. One favorite means of entry is via the weep holes at the bottom of the stucco walls. Some builders here were lax on covering those holes with metal mesh as they should. Former coworker had the damned things coming out of the walls under any receptacle or switch that wasn't totally sealed. Getting the walls foam filled will stop them, but expect it to cost a shed load of money.
As for your girlfriends house, if it was a nest of scorpions then they were the Arizona Bark Scorpion (ABS), most likely imported on palm trees by developers from nurseries in, funny thing, Arizona. Most of the native desert species are solitary, lumbering critters, but the ABS is gregarious, fast, and notoriously aggressive. Given your description of her infestation, they may have been on the building materials. Oh, sometimes people will say "the lumber had scorpion eggs on it". Nope, not happening. They bear live young.
There are several things that will discourage scorpions.
1) Eliminate their food. Yes, this means nuking the foundation and the entire yard with insecticide. I use a Bayer product I can attach on a hose. Get the one that also does for scorpions, although it can be oddly hard to get in Vegas.
2) Eliminate the places where the scorpions and their prey might live. That layer of fist sized lumps of red stone people have as borders against their homes looks wonderful, but it is MegaCity One to the bugs. You may guess how I know.

I had all that stone removed and replaced by smaller gravel, leaving no cover. Don't have stuff stacked up against the outside walls.
3)
Don't have water features, or a pool. You have no idea how many former coworkers' wives gave me the stink eye for that piece of advice.
4) If you have block walls at the property line, consider getting it stuccoed to seal it up remove more hiding places.
5) Hunt the buggers at night one-on-one with a blacklight and some means to kill them. Once I got the population here below a certain level, it seemed to crash hard. If your potential mates are dead, it's hard to breed.
6) When all else fails controlling them in the yard, get the pros in with the cedar spray. They do it at night. The spray flushes the scorpions out (they hate it) and the guys gather them up using a blacklight and long tongs. I know this works because the next door neighbor had it done. Next night I killed nine seeking relief on my side of the common wall.
7) If your efforts don't slow them down in the house, there's little alternative but to bring in the pros and get the home tented.