A friend's child was just diagnosed with angiitis, is that a serious condition?

Potentially. Angiitis is inflammation of a blood vessel wall, or lymphatic vessel wall. There are some serious conditions and complications associated with some kinds of angiitis. It depends on which vessels are involved and where they are in the body.
Yes, very serious. Angiitis is inflammation of the main blood vessels. Since these carry blood away from the heart, inflammation in these can be life threatening. However, effective treatments exist and if the child is treated appropriately there is no reason the child shouldn't recover and lead a normal life. I cared for a patient with severe angiitis and she was able to have children and live normally.