I had angiosarcoma about 10-12 years ago, and was in my upper chest wall . It is, as you probably have learned,
a cancer of the lining of a blood vessel. And extremely rare. Men who have worked in the manufacturing of certain types of plastic are prone to get it. (I am neither!) There are other causes, & I suggest you research it with Google.
Mine started as a very odd-colored bruise that did not hurt, it disappeared, then reappeared in the same place. My dr. said it was nothing, but I insisted she biopsy it. She finally did, and when results came back, said she had never heard of it! I got a new dr.!
I had surgery to remove it, & had no more trouble with that particular cancer. I am one of the fortunate ones. Good luck with your friend, and learn as much about it as you can.