Skin cancer can develop from already existing mole or it can start from normal skin area.
Developing time and appearance of the lesion for skin cancer are usually important values for treatment and prognosis, but, also, there are exceptions when small lesions, looking normal can be very advanced skin cancers in a short period of time.
Usually, it takes months for melanomas to reach a size that is noticeably larger than most other moles (>6mm) without developing serious life-threatening potential, but no one should wait until a mole becomes elevated, or bleeds before seeking medical help.

How is your mother now?
Did she get biopsy and results?