Hi thanks for asking question. Here mostly your friend suffering from acute infection.mostly acute sore throat. Yellow saliva can be due to exhaled cough that might give yellow colour. Here better to do investigation for CBC and X RAY CHEST first for preliminary work up.If in CBC total count high with neutrophilia acute bacterial infection most likely , for that antibiotic has to be taken. It is better to start antibiotic prophylactically even though it comes viral infection by elevated lynphocytes in CBC. Possibilities of pnemonia less, but x ray will rule out it whether any consolidation or pleural effusion present or not. Meanwhile maintain your immunity with more water,fruits and healthy diet. I hope my suggestionn will help you. If still problem then further investigation is done.