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

Can be. Asthma is a chronic inflammatory disease of the lungs that if left untreated can be very serious and potentially fatal. Fortunately, with proper evaluation and treatment most patients will do very well. Like any disease ongoing monitoring and management are required to optimize the patients condition.
Yes. Untreated or poorly treated it can be fatal. Well treated it does not need to have any negative impact on his life or lifespan.
Not usually. Most asthma cases are relatively mild but require attention to the things that may trigger the wheezing. Not very many have severe disease.
Varies. The severity of asthma is variable. If appropriately managed, by a health care provider, the asthma is able to be controlled.
Absolutely! Not treating the underlying allergies that commonly cause childhood asthma means that the lung inflammation is likely to get worse over time, with progressive loss of lung function. All children who wheeze, particularly those over age three, should be evaluated by an allergist for the best possible outcomes. Simply guessing at the most likely allergens is never a good substitute for real treatment.