Sweet... sorry your mom is suffering. Cancer is brutal in that it can steal everything.

Your mom's wound likely isn't healing for a few reasons -firstly and foremost - the cancer is still there and likely in situ - the dr. is right. hopefully chemo will begin to shrink the tumor, and this will allow the hole to heal but it will be a long slow process. Also from a health stand point your mom is very fragile. Nutrition is the building blocks of healing - protein/iron - help her gain strength, but persistent cancer steals her nutrition to continue it's growth cycle. (this is why many cancer patients are so very thin at the end of their life)
Also because her nutrition (strength) is being stolen - it is hard for her body to muster the strength to heal. It's so busy fighting the cancer and keeping all of her basic functions going.

I'm just trying to explain the process to you. This is all ultimately why her wound is not healing. The best they can do is pack it keep it clean and make sure it doesn't become infected, and hope the chemo will help.

Hugs and blessings to you and your mom.