You should never pay to join an agency, they will earn their money from work they find the model.
If its portfolio pictures they need, agencies will usually give you a list of photographers to contact, that they recommend. Then the model would work out a deal with the photographer directly.
If she is having pictures done at the agency or from someone at the agency, you should for sure run. Also its worth finding out the photographers name and looking him up online, to see if he is actually a good photographer or not
