thanks for reaching out. Is your girlfriend working with a doctor who prescribed PEP? If so, I would consult with them, since we can provide information but can't advise you about what you should do (we are not medical professionals).
It sounds like your girlfriend is in the window period from possible exposure, so is doing the right thing starting PEP right away which greatly (but not 100%) reduces the chance of transmission. Since your question is about unprotected sex that happened yesterday, I would speak to your doctor today so that if they do recommend PEP, you can start it within the 72 hour window period.
Here is some more information:
PrEP and PEP : https://h-i-v.net/?p=258
More details on PEP: https://h-i-v.net/?p=260
I hope this gives you a good start and we wish you both the best. Let us know if you are looking for any more information.
Liz (Team Member)