I've developed a second breast lump in a matter of days. is it likely to be cancer?

No: I guess my first question is, what was the first lump? In general, a new lump that appears to develop "overnight" is often a breast cyst, a fluid filled lump that develops after a duct becomes blocked. It is almost impossible to distinguish a cyst from a cancer by exam alone; an ultrasound can determine this easily. Regardless, any new lump in the breast warrants physician evaluation.

No: Lumps arising rapidly may be cysts or hematomas (blood containing masses such as after trauma or a biopsy).

See your doc: You should alert your friend primary care physician as to any changes you detect in your breasts.