My 54 Yo Brother Has A Black Gangrenous Wound On

My 54 yo brother has a black gangrenous wound on his ankle and will not seek tx. It's about a small tennis ball size. Foot was inflamed looking. Now is pale. I would like to know what a "general' course w this might look like. Is there usually a slow progression and decline or is there often a tipping point.? He has lost a lot of wt and is undiagnosed. Just wanting to know what to expect