What Causes Purple Bruises On Nipples?

my friends nipple was going purple on and off for the last few years. just recently they are not just purple (bruising under the skin) but there are dark almost scabs on the outside. they are not scabs though. they are hard dry and brown it looks like dry poo. but when one fell off and he tried to scrape the other ones off he said it hurt. He has already been to a few doctors but this last turn of them going from bruising to an external thing is really freaky. any suggestions as to what this may be or who he should go see to get a diagnosis? thank you