Anybody looking to buy or sell property in Spain would be well-advised to only deal with an estate agent with 'API' status. This is only available to people with relevant degrees (e.g. in law) who have undertaken extensive further study.
However, there are thousands of unqualified people trying to make a living as estate agents in Spain, often without much success.
Anyone planning to set up an estate agency in Spain would be well advised to spend a great deal of time studying Spanish property law. There is plenty of basic information available on the web but your girlfriend really needs to get hold of some good law textbooks and read through them. Unfortunately, she'll probably need to get her Spanish up to at least A-level standard before she'll be able to make much sense out of them.
She'll also need a good knowledge of Spanish financial products, such as mortgages and insurance. (I'm not sure whether Spanish law requires someone to have a licence to sell such products). It might be best to seek out a Spanish financial services firm, who sell their products through estate agencies, to get some advice from them as to what knowledge, qualifications or licences are required.
This page gives some background information. (Also, the links at the bottom of the page look as if they might lead to further useful information):
http://www.thespanishpropertyregister.com/span ish-property-business.htm
Chris