If this lump,round in shape, feels smooth to the touch, and painful, could be a sebaceous cyst resulting from clogged sebaceous gland.
Armpits are common sites for presentation of sebaceous cysts.

Best wishes!
Marija

It could be a sebaceous cyst since it is painful, movable, and encapsulated. A malignant cyst is the opposite. It is not painful, immobile and non-encapsulated. That's when you should see your GP or an oncologist to confirm the matter..

wll i have the same thing under my arm my doctor said that it might have been a cyst, so i had it removed but 3 months later it came back, so now they think that it is a skin condition so i now have to go for another operation to remove all the skin.

so that could be the problem with your friend so thell her to see her GP