Hi thanks for your question.
The swelling which you mentiong on the upper part of thigh and spreading to groin could be a boil (infected hair follicle) .This boil is now spreading leading to cellulitis
(inflammation of surrounding structure).
Since it has pockets of puss inside,you father in law need to see a general surgeon
,who can drain the puss and put him on antibiotics .
Hope this answers your question.