A friend was just diagnosed with gynecomastia, is that a serious condition?

No. It is a benign (non-cancerous) growth or swelling of the nipple and breast tissue in men. There are multiple causes, and it often goes away by itself especially in younger persons. It can be surgically removed if it is a cosmetic issue, but this is not necessary. Alcohol use, marijuana, and steroid use can cause it, but often it just happens by itself.
No. Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. The decision of whether or not to proceed with surgery to correct gynecomastia will ultimately be a very personal decision based on whether the enlarged breast gland causes physical and/or psychosocial “distress”.
No. See my response to same question below. Friend should see doctor in follow up. Can be treated if necessary. Usually goes away by itself. Exercise and weight lifting helps. Be sure it's not a neuroendoctrine disorder.
Not usually. This is quite common in young males and is often temporary during the early pubertal years. If it persists, surgical treatment might be indicated. A work up by a qualified physician is needed.
Not necessarily. There are various medical reasons and medication side effects that can cause this condition. A plastic surgeon will be able to guide your workup based on your history and physical exam. In most cases it is easily treatable with liposuction and minor excision of tissue.
No. Just like some women are flat chested some men have small breasts. If breast growth is sudden or bothersome it should be investigated by your physician to rule out an underlying endocrine abnormality. Very rarely male breast cancer can present as breast enlargement. Often it is associated with weight gain or anabolic steroid use.