Neck lymph nodes can be the result of tonsillitis, pharyngitis (infection of upper throat), infection
in mouth, caries
in tooth, internal
ear infection, outer
ear infection,
eczema, dermatitis, dandruff, breast cancer, fungal infection, other cancers like lymphoma etc. If your friend had flu like symptoms initially then it could be due to infectious mononucleosis too.
The enumerated the possibilities can be investigated or ruled out one by one by your physician, ENT and dentist. Take care!