Roger, if she's doing these in camera then she's come up with a way to adjust the exposure so that, in the end, the exposure is correct in total. Maybe she's using ND filters, I'm not sure. Not having used a Holga I don't know how much adjustability you have but I don't think it's much. If you can adjust the ISO and set, say, ISO 400 when using ISO 100 film and have the exposure settings adjust automatically, that will effectively underexpose each image so that, in total, the exposure is 'correct' at the end.
I did a lot of multiple exposure photography with film and that's what I'd do. If I were shooting ISO 100 film, and wanted to do 8 shots, I'd adjust the ISO to 800. If I wanted 16 shots, I'd set the ISO to 1600. The camera would adjust the exposure to compensate. Once all 8, or whatever number, underexposed images are layered on top of each other, the total is correct. Now, with digital and some Nikon cameras (still the only ones as far as I know that can do ME), the camera will automatically adjust the 'gain' and effectively underexpose each image so that when all are combined, the total exposure is correct. You could do it yourself by adjusting the ISO and turn 'auto-gain' off in the ME menu.
Masking off certain parts of the frame isn't difficult either. Again, this is something that photographers have been doing for many years. It can be as simple as holding a piece of black cardboard over the lens. The black board can be taped on. There are filters available that will block light from half the frame too. Many ways to do this.
If she's scanning the film and combining the images digitally, that's even easier.