How can I help her?

Honestly, this whole thing smells like bulimia or anorexia to me—or maybe it's something else entirely, like they're on some kind of medication or whatever
Kevin Garcia12 figured out.

Joseph Morris5 said:Alright, they aren't actually open wounds, I misspoke there—they're more like red blotches on the skin. It might not even be related, since this just popped up recently, so it could be a rash or some kind of reaction to anything at all, honestly, I don't think that part matters that much...

It’s not "not important"...

If she's dealing with bulimia, she's a danger to herself and needs treatment—whether she wants to admit it or not...
If she's contagious, she's a danger to everyone else and needs treatment—whether she likes it or not...

The only time it doesn't matter is if it's some chronic condition she's already aware of, because at the end of the day, everyone has the right to make their own choices as long as we assume they're "of sound mind" and aren't posing a threat to the public...

Now, "spots" and actual open wounds are two very different things, but even with someone struggling with bulimia, you can get vitamin and mineral deficiencies—though maybe not as bad as with pure anorexia—and those spots could be a result of that, or maybe they're just bruises or an allergic reaction to some supplements since she's training in combat sports...

Bottom line is, without a qualified professional actually looking at this, there isn't going to be any real answer...

Look, if they refuse to see a doctor, then just let them die. I honestly don't have anything else to say to you. I find people who flat-out reject medical treatment absolutely fascinating. And how can you even be sure she isn't HIV positive? You mentioned sores all over her body, constant vomiting, and an immune system that's basically shot.
I don't get why your girlfriend is even letting this happen in the first place?? Does she need to be part of the problem too? If she won't go to a clinic, you should just report her to the facility management. Then she'll be forced to seek help because she'll lose her housing. It’s unbelievable, really.