How Can An Abscess At The Site Of Incision Be Treated?

Sir. My father anisur rahman age 55 was went through by aortho byfemorial bypass in 2006. But after that he had to cut his leg above the knee for an infection in lower leg in 2008. But now he is again infected by wound on the cutting end. Doctor have been removed it by surgery without much considering his blood flow in that leg. but after 15 day of operation the wound is not looking good. Some dry black region has been emerged on the operation area.He also have diabetic problem.