Proper clinical examination helps to evaluate the clinical condition. I understand that she might be having unbearable, severe pain associated with swelling. Also, as you described she is unable to open mouth during pain i.e either she has carious wisdom tooth or with periodontal infection there is a swelling.
Next Steps
Visit dentist for further examination & radiographic investigations to evaluate the condition.
Health Tips
gargle with lukewarm salt water after every meals to maintain oral hygiene &apply ice pack EXTRAORALLY .
please Don't apply any hot EXTRAORALLY.