From what I've learned from this site and the UKBA, you certainly can't get a visa to come to the UK to work in a bookshop....
Possibly she came over to visit, and ended up worked under the table? I can't imagine owners of a small independent bookshop knowing much about immigration rules or asking to see work visas or do background checks, ect, so it was probably pretty easy.
I agree with previous comments, is a little bit fishy, someone who seems intelligent just thinking it's okay to come over here and "live your dreams"