The lump needs testing. They think it IS skin cancer? It could be biopsied, so you would know. It may need what's called an "incisional biopsy" where the lump is removed, but if she came through the emergency spay (I presume for Pyometra?) -she should sail through an anesthetic for the lump.
It might be good for you to know for sure if this is cancer? It all depends on what stage the cancer is at. Some cancers can be cured by surgery alone. Some do need follow-up chemo. But just in case it is cancer, and you do decide NOT to give her chemo, she could still have some good quality of life left yet. With supportive care and treatment.