Tuberculosis (through sensitive
ELISA test), cancers of lung, esophagus, lymphoma, acute lymphoblastic leukemia, cystic fibrosis, and histoplasmosis. A PET scan may be diagnostic. If this does not yield results then yes, a lymph node biopsy may be required (however, before that it is important to rule out tuberculosis).
Rheumatoid arthritis too can cause swollen lymph nodes and fever. Hence, may be she is having a flare of rheumatoid arthritis. This needs to be ruled out.