soap being a wash off product won't really do much other then keep the skin around the area from drying out like a detergent bar might.
after her stitches are out and the incision is closed, a nice easily applied salve made with emu and vitamin e would be my choice. or a lotion made with good oils like emu, shea and vitamin e.
i had a mole removed several years ago that needed stitches inside and out, and after the stitches were removed i made a small jar of 3/4 parts emu ( the good cosmetic grade) and 1/4 part vitamin e and would apply this with a q-tip morning and night. not much is needed and you can't even tell i had stitches unless your up close and personnel and then it is really faint.
i have given this to several friends who have had carpal tunnel surgery, a knee operation and abdominal surgery and they are all really happy with the faint colored scar rather then the pink/reddish one you can have.