Generally working students come out of the existing clientel (aka barn rats) if you are looking for tack cleaning/grooming/mucking type work in exchange for lessons or free rides. If she's looking for more of an assistant trainer (paying) type position, she might try contacting some of the horse colleges for a recent grad.
I have had great results both times I used Yard and Groom. You can put up an ad for 6 months (or longer) at a very fair rate. Lots of room to describe the job and post photos of your facility. Also, you can see other people's ads to help in the wording of your own. Even though it's based in England, tons of people from all over the US read it.
Charter member of the I-Refuse-to-Relinquish-My-Whip Clique
Proud owner of one Lunar acre! (Campanus Crater, The Moon)
The issue I have with Yard and Groom is that since if is British, they force the applicants to enter their age. Something that is not done in the US because of all our equal opportunity laws. And I think while it can be positive, I could see it also being very negative for some people who are older but very fit and capable.