SCCA of the skin is the same and can be just as aggressive as SCCA of the lung. They are intrinsically the same. Having said that, skin cancer is usually caught much earlier and smaller because it is on the skin. If it is small and removed completely and has no bad pathologic feature ie poorly differentiated, vascular lymphatic invasion, positive margin and especially perineural invasion. It is usually not productive or cost effective to do a complete workup.
To your other questions , yes it can come back (and we are seeing more of them) and yes it can be deadly like any other SCCA. The classic case we see is someone with LN and a history of multiple skin cancers. These are extremely under reported because they are very commonly listed as unknown primary and not specifically skin. Also many times they recure in the Parotid glands and are called SCCA of the parotid WHEN REALLY IT IS A RECURRENCE OF SKIN CANCER. Parotid SCCA almost never occurs in the US.
So depending on the size, pathology and even a little the location. She MAY need more w/u and or treatment.
Question; Did the derm offer radiation therapy as an option?? It is just as effective as surgery and is more cosmetic. You do have to come for a series of treatments however.