Are these type of moles common for women? Are they common for most women than men? Is it deadly if cancerous?
A mole can develop anywhere on the body. Some moles can be cancerous but are usually easily removed. Google moles and signs of cancerous moles, see if the pics look similar. I wouldnt get too worried as it may be nothing. Yes it can be completely removed. I have a few moles that have changed over time on me that I have yet to get checked out, it's pretty common.
Can a mole develop under your breast? Can that be harmful to her health or can it be unharmful and be removed?
It's not breast cancer, sounds like it could be a mole, they can change and develop at any time. Since it is changing she should get it looked at.