Stage I Level 4 (Thin Skin)

Hi,
My fiance was diagnosed with Stage I (0.8mm) but level 4 melanoma today. From what I have read this is a really odd combination. The dermitologist says that it is because she has thin skin. The tumor was on the back of her leg and she is 24. I was wondering if anyone knows anyone who has had this same experience and knows more about it.
Also, our wedding date is in early September and we both just started new jobs. How will the treatment affect these things?