Information provided by you says that the lump in gum was present for 2 years and was surgically removed 2 weeks back.
Pathological report shows mitotic index 1/10 HPF. Mitotic index refers to cell divisions seen per 10 HPF (high power fields) and shows the malignant potential of a tissue. In notorious types of cancers, mitotic index often seen is 20/10 HPF. In your brother's lump, a mitotic index of 1/10 HPF is relatively safe. Still immunophenotyping
may be required to further classify the lump in terms of tissue diagnosis.
Please tell your mother and brother not to worry. The swelling is most likely due to the healing process of the surgical wound.