More information about the bump like number of bumps, dimensions, whether the skin above the bump is peeling) might have been helpful.
An allergy is usually a systemic answer by the immune system to present allergens. The present allergen usually does not cause such a severe answer in other people, but in allergic people, the allergy causes a severe immune response to a substance that is not so harmful.
Given the fact that your friend was recently in a tropical country, it is possible that the bump might have been caused by an insect bite. It is hard to say what caused the bump without seeing it. Your friend might visit a dermatologist to detect the cause for the bump. Visiting a specialist for infective diseases as soon as possible, is recommended if your friend also gets a fever before the bump completely disappears in order to check for any infective diseases.
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