Its reassuring that her blood results came back normal. Sometimes, post-viral illness Fatigue is severe and can persist for up to 6-8 weeks.
This syndrome is characterized by severe fatigue, lethargy, widespread aches and pains, and sleep problems.
I would wait at this point, as she has been examined and evaluated by the doctors recently, and also the labs are normal.
But if this persist beyond another week or two she would definitely need further investigations/evaluation.
Her ESR/CRP should be checked (these are markers of inflammation) and should be normal now after the infection has gone. If they are still high; it means there is some inflammation in the body and in that case source should be identified and treated.
I would also suggest an ultrasound of both axilla, and groin to see if there are enlarged lymph nodes.
If there are; she would require further imaging in the form of CT scan of the body to identify any other areas of body having enlarged lymph nodes, which would need biopsy for a definitive diagnosis.
Her vitamin D levels should also be checked. Sub-clinical (with out any symptoms) Vitamin D deficiency is very common and manifests usually in the presence of an acute illness like infection.