What Samantha said is spot on. Short term rentals are mostly advertised by tenants and so are normally advertised "off of market". Public transport in that part of the city is extremely convenient, so don't worry about living 10 blocks or more away. I would concentrate on the SoMa area for affordable housing. SoMa residents are primarily young tech or financial workers and so are most likely to have a need for a short-term roommate.
I would check out Craigslist for sublets and short term rentals and there is also AirBnB.com. I would also suggest that she google areas of the city so she knows what is a safe area and what is not. San Francisco also has a great transit system so travelling to her job hopefully won't be a problem if she lives outside of Union Square.