Rules of Customer Service

1. The customer is always right.
2. If the customer is wrong, see Rule 1.

But since this is a friend and not a customer, it may be a bit different. I would probably avoid words like "obscene" - that's a very emotionally loaded word. Bit if you are uncomfortable making a skirt that is, in your opinion too short, try to tell her that in your professional eye's opinion a slightly longer one would flatter her more. Add that it's always easier to shorten it later if it's too long than to lengthen it if it's too short.

But having said all that.... Gee, I wish MY wife wore miniskirts.